Add vsearch plugin for visual # & * search

Copied from
https://github.com/godlygeek/vim-files/blob/master/plugin/vsearch.vim
which resembles https://github.com/bronson/vim-visual-star-search but
uses the # and * keys directly instead of <leader>*. Allows text in the
current visual selection to be searched for as you would a word with #
and *.