Add communal git configuration

To use this, add this to the end of your `~/.gitconfig`:

    [include]
      path = .git.d/communal

This adds all sorts of good stuff:

- Coloring for the UI and diffs.
- Good push defaults (current only).
- Use vimdiff for merges.
- Aliases that either Mike or Rebecca cannot live without.
- A global exclude file.
- A GitHub alias for remotes (cloning, etc.):
    git clone gh:mertiz-burns/dotfiles.git
- Use short dates when blaming.
- Read in a template for git commits.
- gitsh(1) settings.
- Disable autoupdates in gh.
- Prune branches when fetching.

Whew!