feature request: seeing intermediate translations

[joshuahhh]

There's an option right now to output the final AST to a file. That's great! But there are a bunch of other intermediate forms in the translation process without similar options:

• preprocessed Markdown (output of preprocess)
• Pandoc's AST format (output of pandoc)
• internal AST format (output of parsePandocAST, before transformAST has been called)

Making it easy to see these would help with understanding the living-papers internals, debugging new features, etc.