Support for github enterprise

[from-nibly]

I would like to use this at work and we have our own github server. It would be nice to be able to set the api server

edit: I just noticed that this plugin just passes the options object right to the publish-release package which has support for apiUrl. This get's me passed my roadblock but it would also be nice if it read the package.json files correctly as well, and there was documentation about the rest of the possible options.