Plugin-ify the formats code
Currently, libshout support mp3, ogg and webm, but some use cases require more (in Arkena case, we need aac support). Unfortunately, due to (valid) license/political reason, integrating more formats is blocked, leading to poorly maintained forks (see https://github.com/codders/libshout).
I started working on a branch (https://github.com/gquintard/Icecast-libshout/tree/feature-plugin) to have format plugins. It is not complete, and there are several objections to it (most prominently the use of mimes to identify the relevant plugin). So this ticket is here to track remarks/questions/note about this work.