Added ability to create Misaka with custom renderer

Hello, I wanted the ability to create a custom renderer so that you could add preprocessing and postprocessing to the Markdown rendering. If you use the markdown method it should work the same, but if you use the Misaka() constructor with the optional *renderer*  parameter, then it will use Misaka's *render()* method instead of *html()*. Not sure if you think others might find this useful!