Vim keybindings for jump to next and prev marks.

The functionality should be identical to vim 7.
In normal mode "[`" takes you to the next lowercase mark, "]`" takes you
to the previous. "['" and "]'" work similarly but just take you to the
line of the mark, not the column.

I am not sure if the "_goTo..." functions I added to the motions object are
appropriate.  Maybe it is clearer to just duplicate the code?