Add option to run with authentication

[hasheddan]

For many distributed teams, running a doc server on the private network is not conducive to their workflow. It should be possible to run a doc server that requires authentication. It would be good to support at least Github and Google for auth in the short term.

[KasnocknDave]

I can contribute here. I will add authentication options for:

• Private repos on github.com
• Private repos on self hosted Github

[KasnocknDave]

@hasheddan I have opened a PR for this issue #171