Don't explode if an ignored file gets #included.
Retire our custom C++ tokenizer in favor of slurping up includes within the clang compiler plugin. Fixes bug 924631.

Many other improvements as well:
* Better diagnostic messages for failing HTML workers
* Deployment improvements, including schema version awareness and git hash verification.
* Use Pygments for C++ syntax highlighting. (#156)
* Upgrade concurrent.futures to 2.1.4.