Add RFC 5424, ie. syslog levels

[xmacex]

Hi, thanks for a great package. Encouraged by what the README suggests, may I propose adding the logging levels from RFC 5424 The Syslog Protocol to logview-std-level-mappings? The standard syslog uses that hierarchy of levels, and so does e.g. [Monolog](), a PHP logging framework.

It is the following:

In Emacs, I personally have added to logview-additional-level-mappings the following

("RFC 5424"
 (error "EMERGENCY, ALERT, CRITICAL, ERROR")
 (warning "WARNING")
 (information "NOTICE, INFO")
 (debug "DEBUG")
 (trace)
 (aliases "syslog"))

and I use that e.g. with Monolog, with the following in my logview-additional-submodes

("Monolog"
 (format . "[TIMESTAMP] NAME[THREAD].LEVEL: MESSAGE")
 (levels . "RFC 5424"))