
evidence supports plugins for metadata.  As an example, the vorbis.so
provider is included.  Each provider must at least feature the following
two functions:

  int meta_set_for_file(meta **,char *)

  int meta_get_for_file(meta **,char *)

If your provider does not support editing meta-data, just return NOIR_FAIL
in the first function, and tag all meta-data you return in the second
read-only.

For a provider to be used, it must be listed for at least one mime-type
in evidence.providers.
