Made lume.dostring() use load() if no loadstring()

lume.dostring() now checks if the loadstring() function (deprecated in
lua5.2) exists, and uses load() if it does not.

lume.lua

@@ -245,7 +245,7 @@ end
 
 
 function lume.dostring(str)
-  return assert(loadstring(str))()
+  return assert((loadstring or load)(str))()
 end