Work out how to give the plugin author more help than "here's the file contents, invoke Fantomas yourself"

[Smaug123]

Ideally several possible modalities:

• Myriad's version, where you get raw source and output an AST
• Get an F# AST, output an AST
• Get raw source, output an Oak
• Get F# AST, output an Oak
• Stretch goal: get TAST, output something

This can presumably be done by reflectively inspecting the generator to determine which interface the user has implemented. I don't want to take a dependency on Fantomas in the runner right now, though; maybe we should load it reflectively from the plugin?