Add behavior to allow users to use CM addons that come with LT

This allows users and other plugins to take advantage of [a number of CM addons](https://github.com/LightTable/LightTable/tree/master/deploy/core/node_modules/codemirror/addon) that come with LT after #1772 landed. Some handy ones to try in html are edit/matchtags.js and edit/closetag.js (will be adding these to the HTML plugin on the next LT release). Merging on monday unless there are concerns.