Added "search" architecture

Added "search" architecture

A new "search" architecture was added, managed by SearchHandler.  In
WebThing, OpenUri*() was changed to simply Open*(), and changed to
handle invalid URIs by sending them through SearchHandler.  Search
handlers are configured with a new SearchHandler config directive.
As a proof-of-concept, a GoogleSearch plugin was created that searches
google.

Plugins were modified to suit the new Open*() calls, and Vimish's
command-line handling was changed to be more flexible.