Add authUrl and promptName options.

This is to support enterprise githubs. [Enterprise github api endpoints](https://developer.github.com/v3/enterprise/#endpoint-urls) aren't a domain so it needs more than specify a different domain. So I've opted to add two options one to specifically define the authorization end point and another to specify the name used to prompt for username and password so there is more clarity about what you are authorizing against.
i.e. follow the patterns above, single `var`, comma-first under that, `=` lining up