Add a sphinx specific mode

When doc8 scans rst files with sphinx specific roles
and references it is unable to determine what those roles
are and then complains that these false positives are
actual errors.

To avoid these types of false positives add a option that
can be provided when running on files that may contain these
types of errors to allow them to be skipped/ignored.

Change-Id: Ia6896617534f68e517d5996d53cf2de274507f56