Change default key mappings

The original key mappings conflict with the soft undo keymap (ctrl+u). Also added proper menu items in the preferences.
The tricky thing is finding a key combination that is both in-line with your original keymap and doesn't conflict with the default configurations of other packages (Emmet uses ctrl+e for "Expand Abbreviation"). I chose a key combo that was unlikely to be used by default in any other package.