Extend the Handler interface to report message length and parsing errors.

I'm updating otoolep/syslog-gollector to use mcuadros/go-syslog (and thus
jeromer/syslogparser) in lieu of the current regex-based parser and TCP/UDP
server implementation. One of the things I quite like about syslog-gollector is
the stats: messages received, bytes received, num messages successfully parsed
and num messages that failed to parse. 

AFAICT, in order to collect those stats, my best options is to get 1) info at the
Handler interface. Either that, or 2) I would need to instrument deeper within
mcuadros/go-syslog, which seems less good™. This pull request represents my
attempt at implementing 1.