Allow specifying a config file location

[kieraneglin]

I'm in the middle of a refactor that's trying to move all of my tooling configuration files out of the top-level of my application. Most other apps allow you to specify the location a config file is stored, so I was wondering if there would be interest in a PR that does the same for sobelow.

This would be a backward-compatible opt-in change. The idea is that you could specify --config-file from the command line and it'd use that file instead of the hardcoded .sobelow-conf. As a bonus, this would let people give the config file a .exs extension as that would play nicer with many editors

Let me know if you're interested in that and I'd be happy to open a PR!