0.5.0.0

This release comes with a major API refactor. Key changes:

• Added MMark support, as an alternative to Pandoc
• Allows using arbitrary records to load metadata
  • This replaces the previous complex metadata API
• Added Document type that uses the custom metadata record
• Add top-level Rib import namespace for ease of use
• Remove the following:
  • JSON cache
  • Rib.Simple
• Support for Table of Contents via MMark

Other changes:

• Use type-safe path types using the path library.
• Fix #40: Gracefully handle rendering/ parsing errors, without dying.
• Misc error reporting improvements