added "site" option, new counter-based IDs for inputs

– added "site" shortcode option, and "Site to search" widget field.
both default to site_url() if nothing is passed
– added counter-based IDs to generate_search_form() markup and JS,
enabling multiple instances of simple-search-redirect search forms to
exist on a single page [while keeping document.getElementById instead
of having to use classes or more complex js]
readme tweaks