This commit introduces a new option to the core trigger feature: `ruby`.
It can be defined to run ruby code when the trigger is configured to
fire. If you give the ruby block an env and machine argument, the
defined ruby code can use those variables internally.
This commit adds a new option `abort`, which when configured, will exit
the Vagrant process completely. If set to `true`, it will exit cleanly
with exit code 0. Otherwise, the exit code can be configured.
