editor: Finalize current action group when mode is set.

This simple rule restores correct undo behaviour on most commands (eg.
2d2w deletes 4 words and gets undone with a single 'u')
editor: Add Enter() initializer to mode interface.

Currently only used in Normal mode to finalize current action group.
editor: Update SetMode() docstring.
Merge pull request #34 from dtcaciuc/master

editor: Finalize current action group when normal mode is set.