Refactor package main to enable testing of CLI interface

[mccutchen]

I may come to regret the brittleness and busywork this might add to this project's test suite, but: I didn't like how much untested argument/env parsing logic lived inside the main function, so I factored it out into a testable package.

One thing missing from fully testing the app is the ability to actually run it and test graceful server shutdown. Maybe later.

[codecov-commenter]

Codecov Report

Merging #98 (d440d9f) into main (aaf674e) will decrease coverage by 0.61%.
The diff coverage is 93.45%.

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   99.34%   98.72%   -0.62%     
==========================================
  Files           6        7       +1     
  Lines         911     1018     +107     
==========================================
+ Hits          905     1005     +100     
- Misses          3        8       +5     
- Partials        3        5       +2     

Impacted Files	Coverage Δ
httpbin/cmd/cmd.go	93.45% <93.45%> (ø)

[mccutchen]

Overall test coverage has dropped, but this adds test coverage to a previously untested bit of complex logic.