unescape characters Grep Tool treats as special but Vim as literal

addresses https://github.com/mhinz/vim-grepper/issues/224

One could think about using very magic for the Vim search expression to economize much of this (un)escaping.

Anything left to be done? This is useful, for example, if one searched with vim-grepper for lines that match a regular expression containing \b and then wants to jump to the matches in the quickfix list.

Good evening @mhinz, @jez or @ddeville, would one of you mind taking a look at this PR that should ease searching for the grepped string in the obtained quickfix list? At this occasion, how about @mmrwoods 's one as well?