Add a feature-rich Markdown processor

[pydsigner]

Take advantage of MarkdownIt features and plugins to provide a more complete markdown experience.

• toml frontmatter
• attrs
• tables
• custom containers
• code highlighting with pygments
• optional wordcount
• optional templating/variable substitution
• optional typography substitutions

[pydsigner]

Custom containers should allow of course for control over the tag that's output, but should also have a straight-forward mechanism to customize the processing of container contents.