apply other tag if not match

[kennycyb]

Will be good when format not match, I can apply another tag, e.g.

    type parser
    key_name message
    format /myregex/
    unmatched_tag my.app.unknown

then I can have another chained match:

<match my.app.unknown>
   # do something.
</match>

[tagomoris]

Patches welcome :D