Merge Vim emulation rewrite.

You can try it out by launching ```yi --as=vim2``` and configure it using yi/example-configs/yi-vim2.hs as a starting point.

Tests can be run with ```cabal test```, please read yi/src/tests/vimtests/README.rst for some documentation.

Let's use this pull request as a beta release discussion of vim2. When we agree that it's not horribly broken in some way, we can merge it and call it yi-0.7.0