Allow multiple placeholder in shortcuts.

The single previous %s placeholder for url shortcuts is now removed by
placeholders $0-$9 that make vimb browser more usable and the shortcut system
more flexible.

The $0 placeholder will be removed by all query parameter. So the previous
search engine feature can be build up with this new shortcut feature. For
example use 'dl=https://duckduckgo.com/lite/?q=$0' to define a shortcut for
the great duckduckgo search engine.