Add url_add_param and url_remove_param filters

[andershagbard]

This PR adds url_add_param and url_remove_param filters.

Usage:

{{ 'https://shopify.com' | url_add_param: 'foo', 'bar' }} => https://shopify.com?foo=bar
{{ 'https://shopify.com?foo=bar' | url_remove_param: 'foo' }} => https://shopify.com?

Specific use case:
When using the same snippet for product cards in product recommendations and collections, and you want to add a param to the product.url. In product recommendations, the product.url already has params, and in collections, it does not.

These filters makes it very easy to add these params, without worrying about existing.

Caveats
You can't override existing params. It will just append the param, and make it an "array". This is intended.