added hollow_out

src/hollow_out.scad

@@ -0,0 +1,18 @@
+/**
+* hollow_out.scad
+*
+* Hollow out a 2D object. 
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-hollow_out.html
+*
+**/
+
+module hollow_out(shell_thickness) {
+    difference() {
+        children();
+        offset(delta = -shell_thickness) children();
+    }
+}