Added history functionality to search queries on the plugin page

Added basic history.pushState functionality to the plugin search. A .75s delay occurs before history.pushState is called to allow time for typing. Uses existing pattern where the string after 'plugins/' is used as the state value. Used window.onpopstate event to update the list and field.

I added this mainly because I was tired of having to re-type my search every time I pressed the back button after investigating a package.