Make all extractors safe from exceptions

Previously the ruby extractor had its own rescues, but really all extractor
calls should be "untrusted" from the DB, so just wrap there and remove the ruby
custom checks.
Split up a method
Permit extractors to return file-wide custom metadata
Add file-wide metadata to the docs
Update dependencies

Specifically, bump `backports` gem to fix issue with recent 2.0 ruby
patch-levels.
Merge pull request #96 from eapache/extractor-meta

Extractor meta