Adding support for w3af authenticated scans.

Squashed commit of the following:

commit 7d99f5d2c82d84e144b8f099563367a498207251
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Apr 28 14:44:05 2020 +0200

    Updated extended_generic w3af profile to work with newer w3af. Added some config examples.

commit 644923a660f031dcfcf3947f4b744c70a1c2d467
Author: Anže Žitnik <anze.zitnik@xlab.si>
Date:   Tue Apr 28 11:35:20 2020 +0200

    Added auth_scan profile and changed configure.py to support its config. Imported extended_generic auth plugin from old version.