Failing test for openExternalToBlank

Links with rel=external or target=blank should be opened in a new tab if
openExternalToBlank is set, but currently they are not.
Extract external link logic, alter order of checks

`skipRouting` now gets the first say, and all external checks happen in
the same place.
Accept isExternalLink in options
Remove isExternalLink setting

The function can be overridden directly on Layout if necessary.
Merge pull request #471 from iangreenleaf/external_links

content: protocol in links