Add subword cursors - take 3

Fixes #1982 and an alternative to #3961. Also, hopefully can used with atom/vim-mode#377.

I can't figure out how to go left on the first line when there is only white space, see cursor.moveToSubwordBoundary(), /^[\t ]*$/ isn't working.

I haven't added keymaps yet. But maybe:
```cson
'.platform-linux .editor, .platform-win32 .editor':
  'alt-left': 'editor:move-to-previous-subword-boundary'
  'alt-right': 'editor:move-to-next-subword-boundary'
  'alt-shift-left': 'editor:select-to-previous-subword-boundary'
  'alt-shift-right': 'editor:select-to-next-subword-boundary'
  'alt-backspace': 'editor:delete-to-beginning-of-subword'
  'alt-k': 'editor:delete-to-end-of-subword'

'.platform-darwin .editor':
  'ctrl-alt-left': 'editor:move-to-previous-subword-boundary'
  'ctrl-alt-right': 'editor:move-to-next-subword-boundary'
  'ctrl-alt-shift-left': 'editor:select-to-previous-subword-boundary'
  'ctrl-alt-shift-right': 'editor:select-to-next-subword-boundary'
  'ctrl-alt-backspace': 'editor:delete-to-beginning-of-subword'
  'ctrl-alt-k': 'editor:delete-to-end-of-subword'
```
I would not mind if both were the same, though (`ctrl-alt`).  But Sublime uses just `alt` on Linux/Win, I'm not sure about Mac.