Support usage from the command-line

Coveralls.io supports posting from the local command-line, with a local `.coveralls.yml` file including a private `repo_token` that's given to you with the project.

However, I can't use this functionality with Node, because `node-coveralls` *requires* that I push to Travis.

This seems like straightfoward functionality that should exist; if *only* to test your C.I. setup and build configuration locally before pushing changes to your remote repositories.