Update Mr to v0.12.2 using subtrees

The first 4 commits remove the existing copy of Mr and add it back again using git subtrees. The last 2 commits update Mr to v0.12.2

This means Mr can be merged back in in the future by running `git subtree pull --squash --prefix=packages/mr git@github.com:montagejs/mr.git <version> -m "Update Mr to <version>"` keeping all the local changes (like the specs which have been removed).