Run ppx preprocessors before typing and report their errors gracefully.
Add a shim for #ppx directive for 4.02.

The patch that adds it was accepted, but missed the merge window
for 4.02: https://github.com/ocaml/ocaml/pull/54#issuecomment-42351331
Implement autocompletion for #ppx.
Merge pull request #66 from whitequark/master

Run ppx preprocessors before typing and report their errors gracefully