Implemented Per-Call Environment Variables

[AJMansfield]

Added the ability to specify environment variables per-call.
Resolves #644

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2eb6cf0). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #645   +/-   ##
=========================================
  Coverage          ?   93.04%           
=========================================
  Files             ?       61           
  Lines             ?     9996           
  Branches          ?        0           
=========================================
  Hits              ?     9301           
  Misses            ?      695           
  Partials          ?        0

Impacted Files	Coverage Δ
git/cmd.py	85.17% <100%> (ø)
git/test/test_git.py	98.75% <100%> (ø)

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[Byron]

Thanks a bunch for your contribution, and my apologies for getting to this PR that late!