ocamlfind support in rewriter

It would be very nice to be able to write

``` sh
$ ocamlfind query ppx_tools/rewriter -ppx ppx_deriving.show foo.ml`
```

rather than

``` sh
$ ocamlfind ppx_tools/rewriter \
   -ppx '`ocamlfind query ppx_deriving`/ppx_deriving
   `ocamlfind query ppx_deriving`/ppx_deriving_show.cma' \
   foo.ml
```
