Allow the user to pick an output language

[DavidTanner]

I would like to be able to use the cli to generate typescript output. This change adds an optional language flag to specify which of the existing generators might be used.

[the-jasoney]

Just use @preprocessor in your grammar file. For example: @preprocessor typescript at the beginning of your grammar file lets you use typescript in your post-processors and generates typescript for you. @preprocessor also supports coffeescript.

[DavidTanner]

Awesome. Thank you for the answer. That took a bit of digging to find in the docs. https://nearley.js.org/docs/parser#using-preprocessors