Instead of hardcoding values, use a Config type

[parsonsmatt]

Right now, a ton of things are hardcoded. This is less than ideal. A Config type would be a better way to centralize these points of configuration.

data Config = Config 
  { configMainIs :: String -- ^ Name of the main function to be exported.
  , ...
  }

• Add a Config data type to the library
• For hardcoded values, refer to the Config instead.
• (optional): Refactor the library to use ReaderT Config IO a instead of Config -> IO a as the type of functions.

[xldenis]

I think this can be combined with #12, the name of the import module can just be defined in the configuration.

[xldenis]

As for the config, it makes sense to just use optparse to parse the config value from the arguments given to the preprocessor (see hspec-megaparsec).