Add CommandFilter and TextViewCommandFilter helper classes
Implement diff margin commands using a text view command filter (fixes #8)
Enable the diff margin separately from the selection margin
Make sure the text editor is focused after a rollback
Make GitDiffMarginTextViewOptions a static class
Implement the internal commands "Toggle Compare to Index", "Compare to Index", and "Compare to Head"
Add the Toggle Compare to Index button to the diff tool bar, other placements TBD (fixes #35)