customizable behaviour for activity rendering in the view

I have a project where I dynamically instrument the models for an extremely large project in order to generate activity on all models - which are then surfaced in an admin interface to provide an audit log of sorts.

For this reason it's a little bit of a headache to proactively define the views for every action. These two commits add two features that make this a bit easier for my use-case.

1. The first commit allows it to fallback to text rendering when the fallback => :test is specified. This was the described behaviour in the docs so I assume it's what you wanted - however - I added the :fallback parameter since a few tests rely on the ActionView:MissingTemplate.

2. The second commit allows for a default view to specified, again for situations where one is missing. When :fallback is used here, a string can be passed in which indicates the default partial to fallback to when one isn't there. 

Not sure if either of these are enough value to pull them into the official version - but they're super useful to me, so I thought I'd throw them your way. tests, and README changes included. Cheers.