Optional postfix in canHandle

[amundlindstol]

We at mattilsynet have operated with a vhost mapping that uses a suffix identical to our root site (/mattilsynet):
mapping.a.source = /
mapping.a.target = /site/mattilsynet/master/mattilsynet

Using this method removes the "/mattilsynet" when getting url's to content.
Without being able to use this vhost config, we have to remove "/mattilsynet" every time we get a url from some content.

I propose an optional postfix that can be added to the "GuillotineApiWebHandler URL_PATTERN" through the app config

[amundlindstol]

created a PR to 7.0 branch