Adding flatten only feature without filling forms

[om-nayak]

Adding flatten only feature without filling forms.

@revolunet

When I was trying to flatten some multilayer PDFs using fill_form() (passing datas argument as blank), it is showing a warning(screenshot attached).

I am not sure if #46 would solve this issue.

Please do release the new version of the library as the documentation is updated with drop_xfa argument in fill_form(),
but we cannot use it as it is not released.