Improve support for use as site search

[chrismilway]

The documentation on PyPI includes this line: 'FacetedNav can also be used as an advanced search for your site'. However there is no documentation explaining how, and even once you do know how it's not fully-featured enough to be deployable in a real-world context.
I think the following enhancement would need to be made:

1. Allow the value of a Tal expression faceted to appear in the 'current search' facet
2. Include the result of a tal expression portlet when counting results (currently it doesn't)

Perhaps what is needed is a dedicated site search facet, which will pull in the search string (perhaps without the need to replace the '?' with a '#') and actually display a text field (like a text facet).