Support for RFC 5424-style packets and extended runes in syslog tag via AddAllowedRunes() function

This PR adds two new features to the package:

- Support for RFC 5424-style packets

- Support for runes within the syslog tag that would otherwise be RFC non-compliant.   These runes can be specified with the AddAllowedRunes() function, which takes a string of runes that shall be allowed:

```
	s := syslog.NewServer()
	s.AddAllowedRunes("-._")  \\ Allows dashes, periods, and underscores in the syslog tag
	s.AddHandler(newHandler())
```