Add option to prefer an Email address to a Username (#401)

With some providers the Username is an upstream Unique ID, like fex. in the
case of Google.

When matching this with downstream databases, it's sometimes preferred to use
the email address as the  known identifier.

However, when _mixing_ this with sometimes other sources, like htaccess, which
doesn't have a concept of an email address, it can turn difficult.

This change makes the headers _prefer_ to use the Email address, if such exists,
for the Username identifier when passing data to downstream services.

Defaults to Off.

Signed-off-by: D.S. Ljungmark <ljungmark@modio.se>

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>