OAuth2 support in Shiny

[EricMarcon]

scholar can't be used in Shiny apps hosted by a Shiny server such as shinyapps.io because of an authentication issue raised on Stack Overflow, discussed in httr and solved for googlesheets.
Applying this technique in scholar would be great if not easy (for instance : scholar.shiny would use it).

[jefferis]

Thanks for the suggestion. I'd be happy to review a PR but I don't think I'll have time to address this myself.