Underscore support to move to front of line

Related to this enhancement: https://github.com/atom/vim-mode/issues/168

I'm not too familiar with the spec testing and plan to add that, but I wanted to go ahead and send this pull request to see how off the mark this implementation might be. Originally, it was just going to be a extra mapping of the '^' key, but I learned something new from the issue comments which is that '_' with the prefix count will move it down as well. This is also my first public pull requests, so forgive me for my ignorance.