Extract mappings into re-usable commands

This is useful if you want to create a mapping that uses visual-star-search
behind the scenes, for example with ack.vim you could create these mappings

nnoremap + *:AckFromSearch<cr>
nnoremap - #:AckFromSearch<cr>
vnoremap + :<c-u>VStarSearch<cr>:AckFromSearch<cr>
vnoremap - :<c-u>VPoundSearch<cr>:AckFromSearch<cr>