config: Overwrite config.User when env var GITHUB_USER exists

[elgutierrez]

This PR brings the following changes:

• Small refactoring so the changes can be unit tested
• The env vars GITHUB_USER and GITHUB_PASS will overwrite the ones defined in the configuration

Closes #17

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@66e2ea2). Click here to learn what that means. The diff coverage is 42.85%.

@@           Coverage Diff           @@
##             master    #18   +/-   ##
=======================================
  Coverage          ?   3.2%           
=======================================
  Files             ?      2           
  Lines             ?    374           
  Branches          ?      0           
=======================================
  Hits              ?     12           
  Misses            ?    358           
  Partials          ?      4

Impacted Files	Coverage Δ
github.go	0% <ø> (ø)
godocgen.go	3.24% <42.85%> (ø)

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