Implemented folding commands "za" (toggle fold), "zA", "zR" and "zM"
(toggle/open/close all folds recursively).

Other commands should work with folded blocks, e.g. dd on folded block
deletes whole block.

Folding a block moves cursor to the top of the block. Moving cursor left
or right or unfolding the block again won't restore the cursor position
as it was before folding (in Vim the position is restored).

Change-Id: I9fde30ea9358760419486e092d1032e85f415dc1
Reviewed-by: hjk <qthjk@ovi.com>