We can now set filters as well, which updates the window's URL
Return {} on filters.get() if there're no filters

I prefer returning {} instead of undefined so the library's users don't have to
always test for undefined when using the filters. They're guaranteed that we'll
always return an object.
Add filters#setAndRedirectTo method

It allows the user to set the filters and, instead of updating the current
page's filters, redirecting to another URL.