add path_key config variable to be able to insert read file path into the event

When sending the events to the elasticsearch it''s quite useful to be able to understand what file is the origin of the received event. 
The patch add additional 'path_key' tag to the configuration, to hold the filename

    <source>
      type tail_ex
      read_all false
      path_key path
      format none
      path /var/log/syslog
      tag syslog
    </source>

All the changes are backward compatible as the path is inserted into the event only if path_key is specified.