Add Support For Installation Token Parameters

[emoryruscus]

Add support for passing additional repository ids and permissions when creating a github installation token.

https://developer.github.com/v3/apps/#create-a-new-installation-token

[emoryruscus]

Have worked on this and will be submitting a pull request shortly.

[gmlewis]

Are you sure we don't already have this functionality?

https://github.com/gmlewis/go-github/blob/master/github/apps.go#L194-L215

Oh, I see... we have the endpoint but not the full functionality.

Thanks, @emoryruscus.

[kimtore]

Thank you @emoryruscus for bringing this up. We're currently implementing a service that will issue scoped installation tokens, would be great to have #1238 merged into go-github.

[chughpiyush]

@gmlewis I think this one can be closed.

[gmlewis]

Closing as fixed by #1238.