Add option to configure fly by env

[fliepeltje]

Change Summary

What and Why: Allow config to be set by env vars for most flags.

Let's say you're managing multiple infrastructure setups, it can be be convenient to have certain flags/options set in the environment; this is a "one-size" solution for that; the precedence is still any flags you might specify, but if no flag is specified, we check for the corresponding env var (which is all-caps, FLY_ prefixed stuff).

This is a draft/idea for now - I'd love to have a discussion about whether this makes sense or not :)