Add tokenauth example

[tteeoo]

Basic authentication via username and password for the GitHub API is deprecated and will be discontinued November 13, 2020.
As such, I think It's important to have an example for how to use a personal access token to authenticate.
This new tokenauth example is completely identical to basicauth except it creates a *github.Client using oauth2.StaticTokenSource.

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Codecov Report

Merging #1551 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1551   +/-   ##
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  Coverage   67.92%   67.92%           
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  Files          94       94           
  Lines        8670     8670           
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  Hits         5889     5889           
  Misses       1880     1880           
  Partials      901      901           

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[tteeoo]

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[gmlewis]

Thank you, @tteeoo.
Just a few minor nits, and then we should be good to merge after a second LGTM.

[gmlewis]

Thank you, @tteeoo!
LGTM.

Awaiting second LGTM before merging.

[wesleimp]

LGTM 👌

[gmlewis]

Thank you, @wesleimp!
Merging.