option to prefix every log message before sending

The main reason I've made this change is to make it easy to use remote_syslog to send logs to the logentries service using their token based routing (https://logentries.com/doc/input-token/) It requires each log message to include a token which they use to route your logs when sent to a shared endpoint. 

Sometimes it's easy to add this to a log format (e.g. nginx access logs) but not to others (e.g. nginx error logs)

Other possible use cases, adding hostname to a log.