Added short inserts and M5 inserts.

screw_insert() now takes a short flag.
screw_length() can now be passed an insert object.

tests/inserts.scad

@@ -24,9 +24,15 @@ include <../vitamins/inserts.scad>
 module inserts() {
 
     for(i = [0: len(inserts) -1])
-        translate([10 * i, 0])
+        translate([10 * i, 5])
             insert(inserts[i]);
 
+    for(i = [0: len(short_inserts) -1])
+        translate([10 * i, -5])
+            insert(short_inserts[i]);
+
+
+
     stl_colour(pp1_colour)
         translate([len(inserts) * 10, 0]) {
             insert_lug(inserts[0], 2, 1);