Add ; and , motions

Implemented `;` and `,` motions for repeating the latest `f`, `F`, `t`, `T`, in the same direction and in opposite direction.

http://vimhelp.appspot.com/motion.txt.html#%3B
http://vimhelp.appspot.com/motion.txt.html#%2C

Also, added `f`, `F`, `t` and `T` to the docs, since they were missing and it is related to this PR.
By the way, a heads up in case it should be changed to `vimState` as well: [search-motion](https://github.com/atom/vim-mode/blob/2ec1119b2a34587aae44f774b84a35d9cb207a68/lib/motions/search-motion.coffee#L11).