Add post-compiler for CSP-safe version.

Ported from https://github.com/RReverser/acorn-csp.
Fixes #90.
Fixes #123.

@marijnh Added as per request. Please note that I've added precompiled version of current `acorn.js`, but for further compilations this still requires hook on your side (probably in the same place where you call `update_authors.sh`).