Most awesome string metatable hack

[Andrew Starks <andrew.starks@...>]

```lua

smt = getmetatable("sd")

smt.__call = string.format

print( ("Hello, %s!")("world") )

--> Hello, world!

```

I hadn't seen this hack before. I'm also sure it's been the subject of past controversy, but I think that this syntax is awesome enough for me to forgive lua for omitting printf!

--Andrew