Make plugins out of file formats for flexible build configuration

[trgwii]

There's likely going to be usecases where we want specialized builds of the
ivm-apps "app" with selective file decoding support based on the file formats on
each reel. For that purpose we should move towards a configurable build based on
selected file formats.

We might also want a test harness to make it much easier for external developers
to contribute support for their own file formats. For this purpose we can start
with something similar to the static file demos (Embed files into ivm bytecode
alongside the decoder to test decoders in-vm)