Add ability to change default host

[antonkomarev]

For the testing purposes I need to change https://api.github.com host to the mock API url, but there is no way to do it. All auto-tests should go to the mock service, instead of real GitHub.

We can use enterprise url as a workaround, but it looks like a wrong path.

I see 3 ways to implement this feature:

1. Check in Github\Client::__construct method if AddHostPlugin already defined, and skip it's definition in this case. Set enterprise URL will require the same check.

2. Add method to change host plugin manually in Github\Client object.

3. Make Github\Client::getHttpClientBuilder public.