Support and test envPrefix on Sub

[afirth]

This builds on #1056 and adds support and tests for EnvPrefix using Sub with AutomaticEnv

@TaylorOno commented on Dec 24, 2020
This PR fixes two issues with Sub():
Options were not copied from parent (e.g, automaticEnvApplied).
Environment variable names were wrong: Sub("x").Sub("y").Get("z") should look up env var x.y.z but was instead looking up z

Probably fixes:
#745
#507
#1012 (partial)
#801 (partial)

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[afirth]

@TaylorOno I can't PR into yours as it's a fork, but you're welcome to cherry pick this into #1056 instead if you like. Thanks for your work, I wouldn't have figured this out otherwise.

[afirth]

closed as #1056 has cherry-picked it. Please merge #1056 :D