this is good for a first version, but I wonder if we will ever need plugins that that can run both before and after capturing...  If so we'd need to change this approach (perhaps to return an array).  

My initial thoughts were to simply have plugins implement a method e.g.`def post_capture` and/or `def pre_capture` and those methods would be called (if they had been defined) in all installed plugins by the runner at the appropriate points. 

For most all plugins the existing `def run` method would simply become `def post_capture`.  But I'm open to discuss this approach too.  We ideally need something that is easy to understand, easy to extend and simple enough to convert existing plugins.

We obviously need to get it right first time, getting plugin owners to fix and change their plugins will be time-consuming if we make any changes after releasing this.