Implement new client options

[busunkim96]

Add support for the three new client_options quota_project_id, scopes, and credentials_file.

https://googleapis.dev/python/google-api-core/latest/client_options.html

[tseaver]

I believe this work is blocked on merge / release of googleapis/python-cloud-core#15

[tseaver]

PR needs to pin google-cloud-core >= 1.4.0, < 2.0dev.