Allow setting other service_* parameters

[bmw]

Based on the text at the bottom of https://docs.coveralls.io/supported-ci-services, my understanding is to use Coveralls with an unsupported CI provider, you need to set the other service_* parameters in https://docs.coveralls.io/api-reference.

There doesn't currently appear to be a way to do this though which is preventing me from using this project.

This issue is related to https://github.com/z4r/python-coveralls/issues/23.