Support environment variable interpolation

[arctic-hen7]

This feature could easily be added by allowing the definition of a .env parameter or the like in long-form notation, along with a separate object [env_files] or similar in which the user could define a list of files to pull in environment variables from (in addition to any provided to Bonnie itself on the command line). The system would then fail gracefully if a variable isn't defined.

I think this feature would make developing more complex and integrated systems much easier. For example, if you define Docker ports to be opened in an environment variable file and then interpolate that into docker-compose.yml, if you need to expose those same ports in a docker-compose run command you alias with Bonnie, you should be able to read from that very same file.

[arctic-hen7]

I personally think this should be relatively high priority, especially given it should be reasonably simple to implement.

[arctic-hen7]

All done! Please let me know if there's anything missing!