Prettyfing Describe() for record types.

If a record type has a name and ODesc is set to short, we now print
the name instead of the full field list.
Extending the file analyzer API with MIME types and support for
dynamically instantiated analyzers.

- One can now register individual file analyzers for specific MIME
  types, and the script-level can then activate all analyzers associated
  with a type.

  Note: I'm not fully sure if this is right approach, maybe the core
  should just go ahead and activate analyzers directly once it has
  determined the MIME type. However, the script-layer is doing
  additional state tracking that we'd then need to do differently.

- The file API has been extended to support dynamic sub-analyzers
  along the lines of the protocol analyer API.