Add ability to listen for only specific events

[josegonzalez]

Combined with --only-events, this could make it easy to listen to make slightly more programmable script triggering. For instance, you might not want to trigger your asset pipeline on file create, but only file delete and file updated.

[cespare]

I'm somewhat skeptical of the value of this feature. In the example you gave, why wouldn't you want to invoke your asset pipeline when you create files?

[josegonzalez]

I'll be honest and say that this was so long ago that I don't even remember the use case. I think it had to do with how file syncing worked for a particular system - create events happened regardless of whether it was a create or an update, so updates would result in the system being invoked twice.

Seems like #47 is related?