cli: add support for runtime config files and env vars

[eysi09]

We should support per user and per project CLI runtime configuration files.

User settings could be at $Home/.gardenrc and project settings at /path/to/project/.gardenrc. Among other things, the config files could contain settings for CLI arguments and options. Here's how yarn does it. Other settings could also be added there as applicable.

This would also be a good chance to improve how we handle env variables, it's pretty ad hoc at the moment.

I'm guessing the order of precedence should be:

project settings < user settings  < env vars < cli args 

where cli args has the highest precedence.

[eysi09]

@edvald, was the conclusion to implement this in Go?

[edvald]

That would make sense, since it'll affect behavior at that level.

[eysi09]

See also #668.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[eysi09]

Closing in favour of #1523.