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.