Added function plumed_custom_skip

Can be used to skip a regtest based on some complex test.
E.g., to skip a test on mac add this to the config file:

function plumed_custom_skip(){
  if [ "$(uname)" == "Darwin" ] ; then
    return 0
  fi
}

regtest/scripts/run

@@ -103,6 +103,13 @@ do
   fi
 done
 
+if type -t plumed_custom_skip 1>/dev/null ; then
+  if plumed_custom_skip ; then
+    echo "NOT_APPLIABLE (plumed_custom_skip)"
+    exit 0;
+  fi
+fi
+
 if type -t plumed_regtest_before 1>/dev/null ; then
   plumed_regtest_before
 fi