Inferring output type based on `output` option

[4ntke]

I suggest improving the return type inference. The main function returns a union of type but the return type can be narrowed to the type specified by the value of the output option. By default, a string value is returned, because the default value of the output option is string. Narrowing the return type based on output option would be great.

[avoidwork]

@4ntke open a pull request if you'd like to see a potential change.