Project specific environment variable

This is a followup to an old issue [#157](https://github.com/srushti/goldberg/pull/157).

I've cleaned up the PR and added tests for the changes.

Its useful when we need run same project under two different configurations. For example if we need to run `rake ci` and `foo=bar rake ci`.

This would serve as an additional option to set the environment variable, instead of creating a local goldberg_config.rb for the custom project in the CI box.