Github enterprise support

[andrewhamon]

Github enterprise support

• add a config value spr.githubApiDomain
• collect this value during spr init
• update regex to not expect 'github.com'
• generate PR urls correctly
• adds some test cases
• updates docs

I did not add a CLI flag - it seems like not all options have CLI flags and I don't see much of a use case to change this on-the-fly within a repo. Happy to add it if desired, though.

Test plan: Manually tested on my companies github enterprise instance