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.
