Add relative move and rotate to rel plugin (#794)

The relative position in rel plugin is currently based on the world coordinate. So for the same effect, like fly in from the right-hand side, we must use different `data-rel-x/y/z` value. Why not let the plugin do the hard part?

So I introduce a `data-rel-position`, when set to `relative`, all relative attribute is based on the position and rotation of previous slide. So no matter the rotation of previous slide, data-rel-x="1000" always looks like fly in from the right-hand side. We can change the position and rotation of one slide, and the position of all following slides will be changed too.

When `data-rel-position` is set to `relative`, relative rotation has a clear meaning. It describes the relative rotations between slides. We don't need to set rotations for all slide, setting the key slides is enough. If `data-rel-position` is not relative, the effect of `data-rel-rotate-x/y/z` is not clear, so they're only used when `data-rel-position="relative"`.

After the introduction of relative rotation, there're 6 attribute that will inherit from previous slide. If we want to set a relative X move, we have to set all other 5 attributes to 0. It's boring. So a `data-rel-clear` is used to set all 6 attributes to 0, and then the value specified in current slide is applied. 

The `examples/3D-positions/index.html` shows some usage. As you can see, the html code of two slide ring is the same, and slides except for the first two in a ring has no position attributes. It work by inheriting the previous one.

This PR invokes a lot math calculations. Basically, the rotation of a slide is translated into the coordinate describing the directions of X/Y/Z axes. And `data-rel-x/y/z` can be easily calculated by that. The rotations is the hard part, I mainly use the algorithm in the Quaternions and spatial rotation - Wikipedia to compose two and more rotations.  I'm not a math guy, hope I don't make much mistakes.

src/plugins/rel/README.md

@@ -22,6 +22,13 @@ Following html attributes are supported for step elements:
     data-rel-z
     data-rel-to
 
+    data-rel-rotate-x
+    data-rel-rotate-y
+    data-rel-rotate-z
+
+    data-rel-position
+    data-rel-reset
+
 Non-zero values are also inherited from the previous step. This makes it easy to 
 create a boring presentation where each slide shifts for example 1000px down 
 from the previous.
@@ -47,6 +54,31 @@ If you need a reference to a step that is shown later make use of the goto plugi
     <div id="shown-earlier" class="step" data-rel-to="shown-later" data-rel-x="1000" data-rel-y="500" data-goto-prev="shown-first" data-goto-next="shown-later">
     <div id="shown-last" class="step" data-goto-prev="shown-later">
 
+Relative positioning
+--------------------
+
+All `data-rel-x`/`y`/`z` is used world coordinate by default. So the value should be alternated according to the rotation state of previous slides.
+
+To easy relative positioning, the `data-rel-position` attribute is introduced.
+
+`data-rel-position` has a default value of "absolute", which works like this attribute is not introduced.
+
+When `data-rel-position="relative"`, everything changed. The rotation of previous is no need to be considered, you can set all the `data-rel-` attributes as if the previous has no rotation. This plugin will calculate the acual position according to the position and rotation of the previous slide and the relative settings. This make it possible to split a presentation into parts, construct each parts standalone without consider the final position, and then assemble them together.
+
+For example, if you want the slide slided in from the right hand side, all you need is `data-rel-x="1000"`, no matter the rotation of the previous slide. If the previous slide has `data-rotate-z="90"`, the actual attribute of this slide will work like `data-rel-y="1000"`.
+
+Not only relative positions, the relative rotations can be used while `data-rel-position="relative"`.
+
+For example, `data-rel-rotate-y="45"` will make the slide has an angle of 45 degree to the previous slide. It make it easy to build a circle and do other complicated positioning.
+
+If not set, the `data-rel-position` attribute will be inherited from previous slide. So we only need to set it at the first slide, then all done.
+
+To avoid the boring need to set most `data-rel-*` to zero, but set only one or two ones, `data-rel-reset` attribute can be used: 
+
+- `data-rel-reset` equals to: `data-rel-x="0" data-rel-y="0" data-rel-z="0" data-rel-rotate-x="0" data-rel-rotate-y="0" data-rel-rotate-z="0"`
+- `data-rel-reset="all"` works like `data-rel-reset`, in additions `data-rotate-x="0" data-rotate-y="0" data-rotate-z="0"`
+
+When `data-rel-position="relative"` and `data-rel-to` is specified, `data-rotate-*` and `data-rel-*` will be inherited from specified slide too.
 
 IMPORTANT: Incompatible change
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