Reject documents with xml content-type

The bultin Chrome XML renderer acts when the content-type is xml.
Since there isn't a way to disable it, let's let it do its job instead.
Other documents which end with a known xml document extension but aren't
being served with an xml content-type, will be automatically highlighted
with Sight. For any other cases, one can always reload the URL in
question appending `#ft=xml` at the end.

Fixes #39, #40