Add helper function that pulls configuration values from the environment

[zachgoldstein]

Suggested by @samstav https://github.com/airbrake/airbrake-python/pull/53#pullrequestreview-23619228

Quoting:

Since this adds support for the many available options referenced in the README, it might be time to have a helper function that pulls configuration values from the environment, where any configurable value could be set by an environment variable. This would not only include airbrake credentials (e.g. api key) but also values that are sent in the payloads when notifying of an errors (e.g. environment, app_version, user_email).

I think this is a good idea. First though, it would be great to separate out everything currently env-var related into a separate function.