Provide a better way to add new extra features

[armonge]

Really like the alexa-skill-test-framework, i'd love to use it with our framework https://github.com/mediarain/voxa, which seems like we could right now as it is. However, there are some assertions we'd like to do, like making sure a transition results in a change to X state, that functionality looks like it would belong inside a plugin but the current mechanism in setExtraFeature is not extensible.

Would this be something you're interested in?

[BrianMacIntosh]

Any use you can get out of it would be great! It does seem like a good idea to allow others to easily add extraFeatures. I will look into it. In the meantime, you should be able to do anything you want using the callback property on the parameter object.