Add ability to run scripts on IFTTT events

[todbot]

As requested by Alon via email.

"For example, if I trigger blink using IFTTT, that triggers a light change, at the same time get it to call a shell script on my Mac? This way I will be able to get my machine to not only notify me with a blink , but also react to an event with a custom script."

Technically not hard, but how to make the UI not confusing for everyone else?