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

I'm updating [otoolep/syslog-gollector](https://github.com/otoolep/syslog-gollector) to use [mcuadros/go-syslog](https://github.com/mcuadros/go-syslog) (and thus [jeromer/syslogparser](https://github.com/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 (using `go-metrics`): 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.