Add Gist mapping to vimrc.

- Doesn't require a 1000 line vim plugin
- Depends on a gist command line tool instead.

(cherry picked from commit 63f252edeed55fe773c2a4712629845b8ccf6bba)

Conflicts:
	.vimrc
Simplify gist mapping.

(cherry picked from commit 05b9438574a9b4098671bed3a2dd404bf1dd9cba)