Make ppx_metaquot a source-level utility

[bobzhang]

currently ppx_metaquot.ml is hard coded with Ast_mapper.run_main which makes it can not be reused as a library.

the only way to use ppx_metaquot is ocamlc.opt -ppx ppx_metaquot (it will imply type checking which is a bit annoying)

My use case is that I want to have a command line utilty to do pure source transformation,
so I can check in generated code without imposing dependencies to my users, are you interested in such use case? thanks

[alainfrisch]

If you only want to source transformation without running the compiler, you can use the rewriter tool, part of this package. Otherwise, I'm not against splitting ppx_metaquot into a reusable library and the ppx wrapper; feel free to submit a PR.