Support Env Variables for Options in Server Config

[devinburnette]

Hello again!

We're looking at setting up a shared repository that isn't the default .github repository. I see the option for it in the server config to change where policy bot looks for a shared policy config, but I don't see that as being a currently supported Env variable. We could probably just as easy deploy a config file with this in it, but so far this is the only thing and Env variables are a lot easier for us to manage.

Would you accept a PR to make the server config parser set options from Env before filling the defaults?

[bluekeyes]

Sure, we already do this for some values (the HTTP server options, GitHub configuration, and secrets), but have been pretty haphazard about adding environment variable support for other options. If you'd like to take a pass at adding in missing ones, that would definitely be appreciated, but I'm also happy to accept a PR that just adds the one variable you need right now.