feat: refactor to accept --proxy-key and --api-url

[bcoe]

expose configuration dials and knobs for folks who connect to GitHub through a proxy.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@052c902). Click here to learn what that means.
The diff coverage is 65.59%.

@@            Coverage Diff            @@
##             master     #180   +/-   ##
=========================================
  Coverage          ?   69.85%           
=========================================
  Files             ?       11           
  Lines             ?     1692           
  Branches          ?      147           
=========================================
  Hits              ?     1182           
  Misses            ?      497           
  Partials          ?       13

Impacted Files	Coverage Δ
src/github-release.ts	55.35% <50%> (ø)
src/github.ts	53.66% <62.96%> (ø)
src/release-pr.ts	68.53% <92.3%> (ø)

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