:boom: Migrate to core config

* Replace old menu item with clipboard utility;

  Instead of the unnecessary old `Open Your Overrides` menu item, we now have a
  `Copy Grammar Scope of Active Editor` menu item. The new one simply grabs the
  grammar scope of the currently active editor, and shoves it into the user's
  clipboard. Handy when editing overrides? (Was more handy prior to nested
  style.)

* Rename `getScopeOverrides` to `getOverridesForScope`.
* Migrate from `overrides.cson` to 'Core Config'.