Adding optional CONFIG_PATH env var support

[dolan-a]

This PR adds support for defining the CONFIG_PATH settings through an environment variable. I'm not in love with the env var's name since it is pretty generic, but I stuck with the naming convention of the other variables in env.js

[decyjphr]

Strange - this PR is basically undoing a previous change 1850938 which removed this flag.

[dolan-a]

Thanks @decyjphr for pointing out the other PR. I looked at the comments on #351, and I think my PR is still worth considering. A few specific points to address:

• Probot's suggestion of using .github for config files: As @martinm82 noted, Probot's best-practices docs say to store configs in the .github directory of repos. I think this suggestion is more appropriate for GitHub Apps that would load repo-specific configurations from the repo directly. Since safe-settings instead stores all configurations in an "admin" repo, I don't think this best-practice is as relevant.

• Updating all hardcoded ".github" directory references: The author of feat: allow different admin repo by env variable #351 rightly noted that there are several spots in the code that are hardcoding the config-path directory. I've since updated this PR to use env.CONFIG_PATH instead. I left most of the README as is -- other than adding a note about the CONFIG_PATH env being an optional param.

Ideally we'd have some unit tests to ensure that the repo and sub-org overrides are being loaded correctly when overriding CONFIG_PATH. It wasn't clear to me how best to go about this though, so if anyone has suggestions, let me know.

[decyjphr]

Thanks for the changes.