Improved Env command

Cleaned up code a bit, along with creating a basic command structure so now its
`duckpan env <command> <args>` and commands are `get, help, list, rm, set`. 
@moollaza @mwmiller I was planning to throw a few options for these as well, like `rm --all` or `--clobber` for set (asking a default yes_no if env_variable exists), what do you guys think? perhaps for later? Btw the latest update is awesome, the colors and especially, relieving me of that long list of modules loaded (wanted to change that since forever). Idk about this, but i think we should have a changelog for the user who upgrades duckpan, so that they know the highlights of what's new, perhaps we can throw it along with cache or something and create a command for it? or any other simpler way.
For the case when it doesn't exist, I think we should `emit_error`?