Allow showing source from multiple repos

[aaronjensen]

Related to https://github.com/scoutapp/scout_apm_ruby/issues/141

It'd be great if we could map an additional private repo to a gem so that we could see the source for that in stacktraces.

The majority of our app is implemented in gems that live in a different mono-repository so stacktraces in scout are of limited use at the moment.

Note that because we use a monorepo for the gems, we'd need to be able to specify a repo and a directory in that repo to find the gem source.

I'm sure this is further complicated by needing to know how to map gem versions to refs in the repo, but something could probably be worked out for that (expecting tags or using the sha of an alpha release from git format-ref)