Adding M118 command to send text to serial

Allows the user to send text to the serial console in order to
communicate with a host - sending debuging information or action
commands, for example.  Text must begin with '//' and this is added if
it is not already present at the beginning of the string.

Marlin/gcode.cpp

@@ -150,7 +150,7 @@ void GCodeParser::parse(char *p) {
   #endif
 
   // Only use string_arg for these M codes
-  if (letter == 'M') switch (codenum) { case 23: case 28: case 30: case 117: case 928: string_arg = p; return; default: break; }
+  if (letter == 'M') switch (codenum) { case 23: case 28: case 30: case 117: case 118: case 928: string_arg = p; return; default: break; }
 
   #if ENABLED(DEBUG_GCODE_PARSER)
     const bool debug = codenum == 800;