Adding different search tools (such as ripgrep/ag/etc.) to lnav

[macintacos]

I'd love it if I were able to substitute the current search engine with a tool like ripgrep (which I use on a daily basis in pretty much all other contexts). This would also be a potential fix for the brew installation of lnav, which is dependent on a version of pcre that straight-up doesn't work (see #638).