Add support for setting environment variable in .scuba.yml

[JonathonReinhart]

Closes #88

[codecov-io]

Codecov Report

Merging #120 into master will increase coverage by 0.08%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   94.87%   94.95%   +0.08%     
==========================================
  Files           9        9              
  Lines         566      595      +29     
==========================================
+ Hits          537      565      +28     
- Misses         29       30       +1

Impacted Files	Coverage Δ
scuba/utils.py	100% <100%> (ø)	⬆️
scuba/__main__.py	96.72% <100%> (+0.01%)	⬆️
scuba/config.py	98.76% <96.87%> (-0.49%)	⬇️

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[JonathonReinhart]

@xanarin Would you care to review this?

[xanarin]

Works in all of my target use cases 👍 However, there still appears to be an edge case that isn't handled properly.