Add a framework for synchronously executed preInit and preStepLeave plugins.

This allows plugins to register to be executed at the beginning of
impress().init() and impress().goto() respectively. By returning false,
a plugin can also cancel the event.

Also adds 3 plugins that use this: rel, goto and stop.

src/plugins/rel/README.md

@@ -0,0 +1,89 @@
+Relative Positioning Plugin
+===========================
+
+This plugin provides support for defining the coordinates of a step relative
+to the previous step. This is often more convenient when creating presentations,
+since as you add, remove or move steps, you may not need to edit the positions
+as much as is the case with the absolute coordinates supported by impress.js
+core.
+
+Example:
+
+        <!-- Position step 1000 px to the right and 500 px up from the previous step. -->
+        <div class="step" data-rel-x="1000" data-rel-y="500">
+
+Following html attributes are supported for step elements:
+
+    data-rel-x
+    data-rel-y
+    data-rel-z
+
+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.
+
+The above relative values are ignored, or set to zero, if the corresponding 
+absolute value (`data-x` etc...) is set. Note that this also has the effect of
+resetting the inheritance functionality.
+
+In addition to plain numbers, which are pixel values, it is also possible to
+define relative positions as a multiple of screen height and width, using
+a unit of "h" and "w", respectively, appended to the number.
+
+Example:
+
+        <div class="step" data-rel-x="1.5w" data-rel-y="1.5h">
+
+
+IMPORTANT: Incompatible change
+------------------------------
+
+Enabling / adding this plugin has a small incompatible side effect on default values.
+
+Prior to this plugin, a missing data-x/y/z attribute would be assigned the default value of 0.
+But when using a version of impress.js with this plugin enabled, a missing data-x/y/z attribute
+will inherit the value from the previous step. (The first step will inherit the default value of 0.)
+
+For example, if you have an old presentation with the following 3 steps, they would be positioned
+differently when using a version of impress.js that includes this plugin:
+
+        <div class="step" data-x="100" data-y="100" data-z="100"></div>
+        <div class="step" data-x="100" data-y="100"></div>
+        <div class="step" data-x="100" data-y="100"></div>
+
+To get the same rendering now, you need to add an explicit `data-z="0"` to the second step:
+
+        <div class="step" data-x="100" data-y="100" data-z="100"></div>
+        <div class="step" data-x="100" data-y="100" data-z="0"></div>
+        <div class="step" data-x="100" data-y="100"></div>
+
+Note that the latter code will render correctly also in old versions of impress.js.
+
+If you have an old presentation that doesn't use relative positioning, and for some reason you
+cannot or don't want to add the explicit 0 values where needed, your last resort is to simply
+remove the `rel.js` plugin completely. You can either:
+
+* Remove `rel.js` from [/build.js](../../../build.js) and recompile `impress.js` with: `npm build`
+* Just open [/js/impress.js] in an editor and delete the `rel.js` code.
+* Or, just uncomment the following single line, which is the last line of the plugin:
+
+        impress.addPreInitPlugin( rel );
+
+
+About Pre-Init Plugins
+----------------------
+
+This plugin is a *pre-init plugin*. It is called synchronously from impress.js
+core at the beginning of `impress().init()`. This allows it to process its own
+data attributes first, and possibly alter the data-x, data-y and data-z attributes
+that will then be processed by `impress().init()`.
+
+(Another name for this kind of plugin might be called a *filter plugin*, but
+*pre-init plugin* is more generic, as a plugin might do whatever it wants in
+the pre-init stage.)
+
+
+Author
+------
+
+Henrik Ingo (@henrikingo), 2016