Add 'selfupdate' command

[justone]

Make it easy to update dfm using dfm itself. The current workflow is:

$ dfm remote add upstream git://github.com/justone/dotfiles.git
$ dfm pull upstream master

The selfupdate subcommand should support both merging from the upstream dotfiles master or grabbing the bleeding edge from the dfm repo.

It should also:

• check for uncommitted files that might mess up the merge