feature request: custom analysis function

With our current model, I think it would be pretty easy to support custom list-wise analyses. For instance, the user could do something like this:

```
custom_analysis = lambda lst: [item+1 for item in lst]
pyr.analyze(pyro, analysis='custom', analysis_function=custom_analysis)
```

admittedly a stupid custom analysis but you get the point..think this would be desirable?