Filtering for rack env variables

The core issue is that with [12Factor](http://12factor.net/config) philosophies, database passwords, aws secrets etc. are passed in as environment variables. This means that they would be sent to Airbrake as `cgi_data` when a rake task throws an exception, as long as it was run without an interactive terminal (which deploys tend to be). So concretely, `rake db:migrate` or `db:seed` would send Airbrake our Sendgrid password and our AWS-keys.

I've fixed this by allowing regular expressions for cgi_data filters, and adding default filters for variables containing `secret` or `password`.