A driver is an executable created from a set of OCaml AST transformers linked
together with a command line frontend.

The aim is to provide a tool that can be used to:

	* easily view the pre-processed version of a file, no need to construct a
	  complex command line: ppx file.ml will do
	* use a single executable to run several transformations: no need to fork
	  many times just for pre-processing
	* improved errors for misspelled/misplaced attributes and extension points