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.
It's because we don't want to reverse in subsequent presses of `,`. Only when the direction is different from the last repeat command.

For example, pressing `,` a first time goes backwards if we were searching forwards. Pressing `,` again should keep going backwards (instead of reversing again). 
Also, pressing `;` should actually reverse if the last repeat was `,`.
 
    f(->)   ,(<-)   ,(<-)   ;(->)   ;(->) 

I hope that makes sense.