Add optional support for the Ace editor

- New command line switches: `a'/`--ace', `--ace-theme',
  `--ace-mode', `--ace-keybinding'; the three latter ones only
  make sense when used in combination with the first one.
- Using Ace is completely optional. The old `textarea' editor
  still works just fine.
- The Ace editor in use here is patched with the keybindings for
  saving and navigating to the previous/next search match.
- Tests
Bump the version number