Add new attribute

New attribute is added by this patch.
This has mainly external information related event.

Now added event_id into this. Supposed value is SHA-256 hash created
from arrived line via stream-events. The same one can be generated
if stream-events can be received. So this id could enable to share
information with external services.

Note that class and key name "Misc" is tentative.
I could not give a good name to general purpose feature like this...

So please let me know if you have a good name.