Plugin like interface for your own sound formats

[ericoporto]

From the #101 comment I wrote

plugin like way where I could override a decoder with one of my own or just add a decoder if I implement an API and if the file format matches an existing one then it would use my decoder instead of the built-in one (if it exists)

Besides this allowing me to implement my own libogg interface, this would allow me to override the existing Timidity implementation and write my own, because the existing one also has problems with some soundfonts (see here)