Custom base class

It's nice that Bootstrap and Foundation are supported out-of-the-box, but it seems like it wouldn't be too much more effort just to make the base class and type prefix configurable. I know this could be achieved by subclassing the flash component, but it seems like a good fit for addon-level configuration when there's already type configuration available.

I'd be happy to create the PR if desired.

No files or tests are named. Start by locating the existing addon-level type configuration and the flash component, then compare how Bootstrap and Foundation base classes and type prefixes are handled; done means a documented configuration supports custom values without requiring a subclass.

索引モデルが issue の本文から書いたものです。