Feature request: External grep support

Hi Shigio

Until now I hadn't noticed that global has its own grep implementation.
It turns out that running ‘grep -l’ with a fixed string pattern on the list
of files obtained with ‘global -P’ consumes about a quarter of the time of
the corresponding ‘global -g’ command in a non-trivial code base
(emacs-master, total size of files searched amounting to 79MB).
Modern grep replacements, such as ripgrep (
https://github.com/BurntSushi/ripgrep) are even faster than that. Exact
numbers will obviously differ based on the actual pattern used.
Perhaps grep() could be changed to off-loading to an external tool
(possibly even configurable via gtags.conf)

Thanks,
Marcus