[REQ] Support git repositories other than github.com

Created by: qmuntal

I'm hosting an auto generated go client in an private instance of github enterprise and the generated mod.go file points to github.com instead of my own host.

I would like to be able to configure the host of my git repository using a cli parameter, for example --git-host, in the same way as it is done for --git-user-id and --git-repo-id.

Without this parameter, the only alternatives I have are:

• Manually update the auto generated code, which creates a maintenance problem when regenerating the code.
• Create an custom extension with the hard-coded host, which seem to be an overkill to just modify an url