Edit actual file tags

[Etua]

Fact 1: For now the only individual properties of a file changeable by Sagger are it's position (parent directory) and filename. In order to be a true tagger we need to support adding file metadata. Currently the only limitation of processed file formats is what can fit into fpcalc that is everything ffmpeg does support which is a lot and ideally the chosen solution should have the same capabilities.

Fact 2: It turns out that ffmpeg supports changing metadata.

To check:

• whether there are many naming conventions for tags and do we need to choose one

• whether changing metadata is possible for every processed audio file

• if ffpmeg is going to be used: whether we should use it as a library or once again call the ready binary to do the heavy lifting just as it's done with fpcalc