Function to help configure a GitHub PAT

[jennybc]

Fixes #248 browse_github_pat / use_github_pat

Still a WIP. Currently, pretty faithful to the PR made by @jeroen on devtools.

This should finish quickly, so @hadley, happy to get any opinions you have:

• Name: [create|browse|use]_github_[pat|token]()? usethis functions that deal with the PAT accept it via the auth_token argument (Is that what we want? gh uses .token.)
• Default scopes?
• Default nickname (a.k.a. "description")?
• Shall I immediately message about using edit_r_environ()? Even call it?

I know I need to rationalize PAT docs, check if interactive, use in-house UI functions, etc.

Screenshot:

[hadley]

I think browse_ would be the name most in keeping with the current behaviour.

Do we want to use the API to automate creation even further? Maybe it's worth pushing that off into gh, and considering later as part of an overhaul of how gh does auth?

[jennybc]

Do we want to use the API to automate creation even further? Maybe it's worth pushing that off into gh, and considering later as part of an overhaul of how gh does auth?

Yeah, I am not inclined to get into this with usethis for this release. I'd like to just make the web flow easier for an individual this time.