make a ghc-style preprocessor script

According to this:
http://www.haskell.org/ghc/docs/latest/html/users_guide/options-phases.html

You can ship a script:

```
#!/bin/sh
alex "$2" -o="$3"
```

And then people can use a file with a pragma:
`{-# OPTIONS_GHC -pgmF alex-ghc -F #-}`
without having fun with indirect .x files and resulting .hs files

drift already uses it, and derive possibly will:
https://code.google.com/p/ndmitchell/issues/detail?id=513&colspec=ID%20Status%20Proj%20Summary%20Priority%20Stars