Add switch unused feeder(s) off

Having the non-active feeder motors powered on all the time is not
necessary. A feature to deactivate the unused feeder motors has been
implemented. The feature is enabled on default but can be switched off
in the configuration.

Marlin/Configuration.h

 #define DISABLE_Y false
 #define DISABLE_Z false
 #define DISABLE_E false // For all extruders
+#define DISABLE_UNSELECTED_E true //disable only not selected extruders and keep selected extruder active
 
 #define INVERT_X_DIR true    // for Mendel set to false, for Orca set to true
 #define INVERT_Y_DIR false    // for Mendel set to true, for Orca set to false

Marlin/planner.cpp

   if(block->steps_z != 0) enable_z();
 #endif
 
-  // Enable all
+  // Enable extruder(s)
   if(block->steps_e != 0)
   {
-    enable_e0();
-    enable_e1();
-    enable_e2(); 
+    if (DISABLE_UNSELECTED_E) //enable only selected extruder
+    {
+      switch(extruder)
+      {
+        case 0: enable_e0(); disable_e1(); disable_e2(); break;
+        case 1: disable_e0(); enable_e1(); disable_e2(); break;
+        case 2: disable_e0(); disable_e1(); enable_e2(); break;
+      }
+    }
+    else //enable all
+    {
+      enable_e0();
+      enable_e1();
+      enable_e2(); 
+    }
   }
 
   if (block->steps_e == 0)