Deprecate githubToken in favour of githubAuthToken

[alejandrohdezma]

The githubToken setting has been deprecated in favour of githubAuthToken.

This will probably mean nothing for most of the users, if the default configuration is being used (using a GITHUB_TOKEN environment variable), since the same default configuration is being used for githubAuthToken as was used for githubToken.

The only noticeable change is the type. githubToken had type Token while githubAuthToken has type Option[AuthToken] which allows for directly recover environment variable...

githubAuthToken := sys.env("GITHUB_TOKEN").map(AuthToken)

...while also provides a better integration with other projects, such as sbt-microsites

[codecov]

Codecov Report

Merging #184 into master will decrease coverage by 0.99%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   87.35%   86.36%   -1.00%     
==========================================
  Files          21       21              
  Lines         174      176       +2     
==========================================
  Hits          152      152              
- Misses         22       24       +2     

Impacted Files	Coverage Δ
...m/alejandrohdezma/sbt/github/SbtGithubPlugin.scala	0.00% <0.00%> (ø)
...ejandrohdezma/sbt/github/http/Authentication.scala	66.66% <0.00%> (-33.34%)	⬇️

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