mmcount: message modification plugin which counts messages

This module provides the capability to count log messages by severity
or json property of given app-name.  The count value is added into the
log message as json property named 'mmcount'

Example usage of the module in the configuration file

 module(load="mmcount")

 # count each severity of appname gluster
 action(type="mmcount" appname="gluster")

 # count each value of gf_code of appname gluster
 action(type="mmcount" appname="glusterd" key="!gf_code")

 # count value 9999 of gf_code of appname gluster
 action(type="mmcount" appname="glusterfsd" key="!gf_code" value="9999")

 # send email for every 50th mmcount
 if $app-name == 'glusterfsd' and $!mmcount <> 0 and $!mmcount % 50 == 0 then {
    $ActionMailSMTPServer smtp.example.com
    $ActionMailFrom rsyslog@example.com
    $ActionMailTo glusteradmin@example.com
    $template mailSubject,"50th message of gf_code=9999 on %hostname%"
    $template mailBody,"RSYSLOG Alert\r\nmsg='%msg%'"
    $ActionMailSubject mailSubject
    $ActionExecOnlyOnceEveryInterval 30
    :ommail:;RSYSLOG_SyslogProtocol23Format
 }

Signed-off-by: Bala.FA <barumuga@redhat.com>
add minimal doc for contributed mmcount module