Implement support for exposed and unexposed hooks

[Jim Baker]

Upon a service being exposed, the exposed hook will be run, if it is present in the formula.

This may be an appropriate place to run the open-port command, however, it is up to the formula author where it should be run, since it and close-port are available commands for every hook.

Likewise, when a service is unexposed, the unexposed hook will be run, if present. Many formulas likely do not need to implement this hook, however, it could be an opportunity to shutdown unnecessary processes or remove other resources.