Add openFilters configuration option

[vanbroup]

The default behavior of the filter display is to show values only when there is one filter with six or fewer values. This PR adds a configuration option called openFilters that allows users to include a list of filter names that will open by default, regardless of the number of filters or values present.

new PagefindUI({
    element: "#search",
+    openFilters: ['Tags','Type']
});

[bglw]

Nice one! Thanks for the contribution 😄