allowing tags with more than 32 chars in rfc3164

Hi, 

First at all, congratulations for this excellent syslog parser.

I am implementing a small library for make a [syslog server](https://github.com/mcuadros/go-syslog) as dependency for another project.

I detected that all syslog servers (osx syslogd) not follow the limitation of length in the tag, so i decided to send this pr to enable a way to remove this limitation on-demand. 

If you dont like it my implementation just tell me and i will change it.

Regards and thanks

```
<31>Dec 26 05:08:46 mcuadros-air com.apple.metadata.mdflagwriter[296]: Handle message /Users/mcuadros/Library/Application Support/Sublime Text 3/Local/Auto Save Session.sublime_session
```