Use X as visual mapping for Exchange

As suggested by Drew Neil in [today's VimCast](http://vimcasts.org/episodes/swapping-two-regions-of-text-with-exchange-vim/):

Use the visual mapping `X` to avoid the delay after `vc` when changing visually selected text. Shadowing `v_X` should be no problem as `v_D` does the same and seems to be the more natural choice to delete the visually selected lines.

This fixes #14.