Add support for GitHub Enterprise

[fwouts]

No description provided.

[7loro]

@fwouts Please refer to #1049

• baseUrl for github enterprise should be changed (api/v3)
• graphQLEndpoint should also be changed

[fwouts]

Thank you @7loro! I've merged your PR into this one :)

[7loro]

Thank you @7loro! I've merged your PR into this one :)

Thanks. I think readme should be also updated.

[fwouts]

Already done! Is there anything else to update?

[7loro]

Already done! Is there anything else to update?

Nope! Thanks for making a great extension.