Import from direnv

[DanCardin]

Suggested by someone to me:

I'd like to use it more, but I have a lot of inertia behind .envrc files What would be awesome is if you could write a migration tool

In a world with https://github.com/DanCardin/sauce/issues/31, you could probably just transplant the contents of the .envrc into sauce and call it a day.

More "idiomatically" in sauce-land, we'd be better off trying to extract export commands into env vars. This would necessarily be best-effort. and probably shouldn't be default, since it would ultimately be doing very basic line-wise parsing to find exports.

Probably likely to be tripped up by multiline exports, quoting, and various other things.