Added setting to control visibility on non woocommerce pages

Simply added one propably useful setting value. Control the visibility of the cart widget on pages not related to woocommerce, like blog and frontpage.

I would assume that companies, whose shop is only a subpage on their websites, do not want the cart to be shown on pages not related to the shop.

Added this setting because I needed it.

Using is_woocommerce() as visibility condition value, if the setting is set to 'no', meaning that the cart should not be shown on other pages, than woocommerce pages.