Add url parameter in set_github_pat()

[jeroen]

Many open questions:

• Should we support usernames in the url? Not all managers allow this, GCM-core always overrides the username. Update: gabor suggests using GCM_AUTHORITY=Basic.
• Should we slugify the username? I think not, so that I can do set_github_pat(url = "https://[email protected]") and have that as my new GITHUB_PAT.
• Currently we require the full API in the url, so the user needs to specify like: set_github_pat(url = "https://github.ucla.edu/api/v3/"). Alternatively we could let the user just specify the hostname part from the github enterprise docs because everything else is fixed.

[jennybc]

As for supporting and slugifying username ... we could NOT support that initially, in order to make it easier to move forward now.

Getting support for non-github.com deployments feels like 90% of the win here and we can wait and see if people actually ask for username support.

[jennybc]

For the Windows Git Credential Manager, the GCM_INTERACTIVE environment variable is very intriguing to me:

http://microsoft.github.io/Git-Credential-Manager-for-Windows/Docs/Environment.html#gcm_interactive

Specifies if user can be prompted for credentials or not.

Supports Auto, Always, or Never. Defaults to Auto.