Add option --exceptions-file

This allows a per-project exceptions file.

Once this is released, the current static file list can be moved
to an openstack-manuals specific file.

This is needed to ignore some files in the database-api repository, see
https://review.openstack.org/67716 and
https://review.openstack.org/#/c/67585.

Change-Id: Iab1c7b11680d9785b202b80fa80805b2069ea896
Related-Bug: #1270267