Added CustomParameter method to SearchDescriptor and MultiSearchDescriptor

Added CustomParameter method to SearchDescriptor and
MultiSearchDescriptor - this allows for custom querystring parameters to
be added to URL passed in to search for various purposes. For the
purposes of the project I'm currently working on, I am passing in the
end-user's search term, which by being passed in as a custom querystring
parameter, can be logged to the web logs for future analysis.