ijkl support

I'm a heretic and remap everything from `hjkl` to the arrow-key-shape `ijkl`. This patch lets anyone else who does the same easily use targets.vim comfortably in visual mode by setting `let g:alternate_visual_inner_key = 'h'`, and does nothing if that option isn't set.

Since there's not a lot of people who do that I'd understand if you don't want to merge this, just figured I'd throw a request since it'd be easier than maintaining my own fork.