Fixed #20034 - Ability to retrieve variables in multipart even before parsing files

This is about Trac ticket no. 20034, alters pull request for #898, verifies it, adds tests and documentation.

The new method `variable_complete`, if supplied by the file upload handler, works similarly to `file_complete` and allows to raise `StopFutureHandlers` if future handlers should not be allowed to parse the variable.

This is my first PR to Django. Please verify if there is something lacking in it. Thanks.