Allow RIPGREP_CONFIG_PATH to be a list of files

Bassically, it would be great if I could put:

export RIPGREP_CONFIG_PATH=~/.ripgreprc:./.ripgreprc

(paths are seperated by :s as is "standard"?)

in my shell profile, and then ripgrep would use all the options I have in my global ~/.ripgreprc, but allow me to override them on a per directory basis with a local .ripgreprc (e.g. to add folders to exclude).

And of course it should only give an error if none of the files were found.

My current solution is to have a local ./rg script when I want to add extra options to so it's not a big deal.

This looks like a fairly easy feature to add, so if you like it, I'll submit a pull request.