Implement custom keyboard shortcuts support with a user key map file.

Will add JSDoc later.
@RaymondLim I tried creating the wiki page at: https://github.com/adobe/brackets/wiki/Key-Bindings

But I added "User" to the first header in page, so url was automatically changed to: https://github.com/adobe/brackets/wiki/User-Key-Bindings

Are you ok with changing the url here, or should I try to change it in the wiki? I'm not sure how to delete wiki pages.