Add scroll cursor commands.

Fixes #103. Included commands:
- [z enter](http://vimhelp.appspot.com/scroll.txt.html#z%3CCR%3E)
- [z t](http://vimhelp.appspot.com/scroll.txt.html#zt)
- [z .](http://vimhelp.appspot.com/scroll.txt.html#z.)
- [z z](http://vimhelp.appspot.com/scroll.txt.html#zz)
- [z -](http://vimhelp.appspot.com/scroll.txt.html#z-)
- [z b](http://vimhelp.appspot.com/scroll.txt.html#zb)