Accept list of enabled environments

[zzeni]

Hi. You probably are aware, that sometimes the app might be using different environment for testing (e.g. :test, :ci, :e2e, etc..). Therefore, it could be really useful if there's some global configuration in Hangar, where one can set a list of allowed environments. The default might be [:test] and of course :production can be always ignored.

Please let me know if such a feature seems reasonable to you. Thanks!