Changed the bookmark completion (#27).

If completion is started without any tags, all bookmarks without tags are
completed. If tags are given, only those bookmarks are matched that have all
given tags set.
Fixed non working 'open in new window' from right click menu (#29).

I'm not happy with this implementation. At the moment I can see any other way
than creating a new webview and wait until it receives it's new URL and than
to destroy the webview to use the common logic to open a new instance of vimb.