Implemented Branch Checkout (#141)

[nikewall]

There are three bindings: CBB, CBF, and CB-. CBB is the equivalent of 'git checkout -b'; CBF is the equivalent of 'git checkout -B', and CB- is the equivalent of 'git checkout -'.

I adapted the UI based off of the proposed UI in the issue by changing from autocompletion(?) of typed branch names to listed branches (remote and local alike) which can be selected with CBB/CBF, along with the option to Close the section as well as create a new local branch.

Inline help and vimagit.txt are updated with details.

Airline 'modes' should work, based on what I can tell, but I cannot confirm it since I do not have airline installed in my config.

EDIT: Sorry for the weird extra commits. Needed to amend a comment-change and the remote did NOT like it.

[ThePrimeagen]

@nikewall perhaps assign to someone?

[k0va1]

@jreybert wdyt about this feature?

[jreybert]

I will give a look next week.