first take at addressing performance issues present in xml/html documents

This update tries to address issues #55.
The extension now directly uses the code mirror addons that ship with brackets for code folding. Updated minimum supported version to 38 since global CodeMirror object has now been removed from the extension. Users of edgecode should use [tag 0.2.7](https://github.com/thehogfather/brackets-code-folding/tree/v0.2.7).