Disable evaluation of buffers during a 'vimgrep'

When using 'vimgrep', for each file that is being evaluated, a BufRead event is
fired, cuasing vim-sifnify to evaluate the buffer by calling 'sy#start'.

By using a lock we can reduce the time spent 'vimgrepping' significantly. E.g.
In one of my runs it went from 30+ seconds to < 1 second.

Note that this issue could also have been solved with the autocmd:

    :noautocmd vim /query/ **/*.ext

Yet this has the negative effect of disabling colouring and it is more work to
type.

This fix is based on the comment from this thread:
http://vim.1045645.n5.nabble.com/vimgrep-and-BufRead-td1177993.html

And was taken from the MRU plugin, also referenced in the above thread:
https://github.com/vim-scripts/mru.vim/commit/3e95376cc9f44e4dcb82e35d783ea3c79f1ae6c5

This fix probably also makes #91 less likely to occur.