Add alternative configuration loading

[rmccue]

Right now, the runner loads wp-config in. With the new hook system (#23), this is going to overload wp-config a little. In addition, wp-config loading already doesn't work with some complex systems (like wordpress.org).

Proposal: add a CAVALCADE_CONFIG environment variable which specifies a PHP file to include instead of wp-config. The WP path would still be required on the command line, so the config acts as system-level configuration which can work with multiple installs. (For single install systems, it can replace wp-config instead.)