[WIP] git-drs stand-alone

[kellrott]

This refactors git-drs into a stand alone git plugin, rather than a git-lfs transfer plugin. It adds:

git drs install - Adds new filter block to ~/.gitconfig:

[filter "drs"]
	clean = git-drs clean -- %f
	smudge = git-drs smudge -- %f
	process = git-drs filter-process
	required = true

git drs track - Sets .gitattributes to use new filter:

data/** filter=drs diff=drs merge=drs -text

Then to respond to these commands, there is a git drs clean, git drs smudge and git drs smudge.