Highlight Selection Matches

I added a new preference to enable the `match-highlighter` addon from CodeMirror. When enabled and when a word is selected, it will highlight all the matches in the file. Similar to many other editors. [CodeMirror Demo](http://codemirror.net/demo/matchhighlighter.html)

I used the same style as in the CodeMirror demo, but maybe @larz0 might want to change it.

cc @redmunds since you merged a couple of this CM addons additions.