Add addon/selection/escape-reduce-selections.js

As a part of the multiselect demo, the escape key is described to deselect all selections but the "primary" selection. So this addon adds the ability to deselect all but the top-most selection (Which is the way it functions in Sublime Text)

I pushed my changes into a gh-pages branch so you can [test the functionality in the multiselect demo](http://zombiehippie.github.io/CodeMirror/demo/multiselect)