allow plugin to be manually specified

@caotto @sklum Are you guys ok with the following generalization? It allows the ability to override the file extension when determining which plugin to use. @bklare and I would find this convenient in circumstances where the only other option would be changing the file extension. In our case `.txt` files could conceivably be one of many different formats, and we'd like the ability to explicitly specify which plugin to use in this ambiguous case.