Update GitGutter after writing to the index.
Add GitGutter toggle similar to other git shortcuts.

All of the other shortcuts are related to fugitive, but I feel that it's best to
just put the "git-related" shortcuts together. <leader>gg shouldn't conflict
with any git commands other than grep, which probably much less frequently used.
Merge pull request #356 from johntyree/GitGutter

Update GitGutter after Gwrite