Add github-domains config value for GitHub Enterprise setups, fixes #728

Signed-off-by: Gennady Feldman <gena01@gmail.com>
Updating ConfigCommand so that we can set github-domains from the command line.
First working version of GitHub Enterprise API.
Fixing broken unit test.
Validate the originUrl against the list of 'github-domains' from the config.
Refactoring my code to use a protected function to generate API Url.
Adding Config as parameter to the Driver::supports(), updating all drivers, user and tests.
Per request from Jordi Boggiano (Seldaek) making Config 2nd parameter in supports()
Merge pull request #2375 from gena01/github-ee

Fixing #728 Adding basic support for Github Enterprise