Allow overriding 'commentstring' with b:commentary_format

Closes #23.
Add callback after commenting

Closes #25.
Document b:commentary_format
README tweaks
commentary.vim 1.2

* Switch official interface from \\ to gc. \\ still works.
* Allow overriding 'commentstring' with b:commentary_format.
* Add User CommentaryPost callback.
Allow disabling backslash maps

Closes #24.
Deprecation warnings
Don't trigger modelines after commenting

Closes #26.
Drop \\ maps from docs
Comment text object

Closes #17.
Fix count with gcc
Let Commentary be used as a command

This lets us do things like `:g/TODO/Commentary` or `:.,'aCommentary`

Closes #28.