#41: repeat-motion via s/S, like clever-f

As always, if you have mapped to something other
than s/S, it will work as intended.
fix VSneak mapping
update tests to handle "clever-s" feature

caveat: CursorMoved is not triggered in scripts/macros, so we must force it.
    https://github.com/junegunn/vader.vim/issues/2
make sneak#wrap() more convenient
update documentation for clever-s behavior
prevent 'Z' in target_labels if it is mapped
add tests for clever-s feature closes #41