Allows conditional partial mapping to object list

Hey,

I've made two small changes to allow you loop over an object list and conditionally set which partial should output - useful for feeds where the items may have completely different content or layout

1) I moved the "guts" of _generate_output() into a new function called _parse_html()

2) created a new function called multi_partial() that will read a collection of objects, check if a map_ function was created in the Example_presenter class and collect the name of the partial needed fro output

I added on to the README to explain it's use

Hope that works for you - let me know if you'd like anything changed

Jeff