Customised print_log

[ggodart]

This requests renames print_log to print_log_simple and creates a new version of print_log that;

1. adds optional parameters severity and source
2. if the implementation has a print_log_private subroutine, this is called instead of print_log_simple allowing the implementation to route messages e.g. based on severity. Otherwise print_log_simple is called with the message concatenated with severity and source. Message has to be first to preserve print_log("logfile=myfile.log ...") functionality.
   Backwards compatibility tested.
   Docs updated at https://github.com/hollie/misterhouse/wiki/Developer-help#logging and https://github.com/hollie/misterhouse/wiki/Functions#print_log-print_log_simple-print_msg-print_speaklog (note you may need to scroll here as there appears to be a bug in some browsers).
   An example print_log_private is provided in code/examples