Support branch descriptions

[dbushong]

One often names branches after issue numbers or something else, which is a good practice for long-term reference, but doesn't help one pick apart which branch is about what at-a-glance.

Proposal:

1. Make start take an extra optional description arg: git wf start <branch-name> [description]
2. Store it in .git/config by calling: git config branch.<branch-name>.description <description>
3. Display it at various times when it would currently only print the branch name.
4. Offer something like git wf branch that is an enhanced version of git branch - maybe only lists wf-managed feature branches and includes their descriptions?

Note that this is the "standard" place to store a branch description in git; it's the one that git branch --edit-description operates on. Currently it's not used for a lot inside of git itself, but maybe that will change?