Make convert handle source and target filetypes

Right now the `convert` plugin only converts to one format. I think it would be useful to be able to specify a target format based on the input file's format. For example, I want to convert `.alac` and `.wav` to `.flac`, and `.dff` to `.dsf` (which is not properly handled by beets yet, but I'm working on getting it into Mutagen to eventually let beets handle it). I'm sure there are other cases as well, depending on what your intended player handles.