Added CustomParameter method to SearchDescriptor and MultiSearchDescript...

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.

Though perhaps somewhat out of scope, hopefully this feature is helpful enough to others to be included in main solution.