Add support for M300: Play beep sound

Marlin/Marlin_main.cpp

 // M220 S<factor in percent>- set speed factor override percentage
 // M221 S<factor in percent>- set extrude factor override percentage
 // M240 - Trigger a camera to take a photograph
+// M300 - Play beepsound S<frequency Hz> P<duration ms>
 // M301 - Set PID parameters P I and D
 // M302 - Allow cold extrudes
 // M303 - PID relay autotune S<temperature> sets the target temperature. (default target temperature = 150C)
 float min_pos[3] = { X_MIN_POS, Y_MIN_POS, Z_MIN_POS };
 float max_pos[3] = { X_MAX_POS, Y_MAX_POS, Z_MAX_POS };
 // Extruder offset, only in XY plane
-#if EXTRUDERS > 1
+#if EXTRUDERS > 1
 float extruder_offset[2][EXTRUDERS] = { 
 #if defined(EXTRUDER_OFFSET_X) && defined(EXTRUDER_OFFSET_Y)
   EXTRUDER_OFFSET_X, EXTRUDER_OFFSET_Y 
 #endif
-}; 
+}; 
 #endif
 uint8_t active_extruder = 0;
 int fanSpeed=0;
       }
       
     }break;
-    #endif // FWRETRACT
+    #endif // FWRETRACT
     #if EXTRUDERS > 1
     case 218: // M218 - set hotend offset (in mm), T<extruder_number> X<offset_on_X> Y<offset_on_Y>
     {
          SERIAL_ECHO(extruder_offset[Y_AXIS][tmp_extruder]);
       }
       SERIAL_ECHOLN("");
-    }break;
+    }break;
     #endif
     case 220: // M220 S<factor in percent>- set speed factor override percentage
     {
     }
     break;
 
+    #ifdef BEEPER
+    case 300: // M300
+    {
+      int beepS = 1;
+      int beepP = 1000;
+      if(code_seen('S')) beepS = code_value();
+      if(code_seen('P')) beepP = code_value();
+      tone(BEEPER, beepS);
+      delay(beepP);
+      noTone(BEEPER);
+    }
+    break;
+    #endif // M300
+
     #ifdef PIDTEMP
     case 301: // M301
       {
         if(make_move && Stopped == false) {
            prepare_move();
         }
-      }
+      }
       #endif
       SERIAL_ECHO_START;
       SERIAL_ECHO(MSG_ACTIVE_EXTRUDER);