#9 audiogen preview

Enhance the audiogen system to support a `preview` method. This is similar to generation, but will return only a description of what will be done. Specifically, a preview should do everything except modifying filesystem or other part of the environment. The `preview` method will be optional, indicating that audiogens which have no such method do not have a preview. The preview should then be displayed on the webpage that gives information about an event. Warning: currently the audiogen is a simple function, so adding a `preview` function to it requires a bit of refactoring.