Add basic support for loading configuration from the environment (#139)
All GitHub configuration and certain sensitive values in the Bulldozer configuration can now be read from environment variables. Values in the environment override values set in the configuration file. The variables are described next to the matching configuration property in the example configuration.
Fix handling of check_run events (#142)
GitHub does not always include a full pull request object in webhook payloads for the check_run event. Always use a full object loaded from the API for processing instead.