WIP: Autolink packages in dependency files like `Gemfile`

@DouweM has taken over this merge request, and writes:

Supports:

• Gemfile (Ruby)
• *.gemspec (Ruby)
• package.json (Node.js)
• composer.json (PHP)
• Podfile (Objective-C)
• *.podspec (Objective-C)
• *.podspec.json (Objective-C)
• Cartfile (Objective-C)
• Godeps.json (Go)
• requirements.txt (Python)

To do:

• Make sure blob rendering performance doesn't suffer dramatically
• Don't run pre-processing pipeline, because this may break the highlighting

The link colors used are directly taken from the existing color palette for those themes.