url.update should support a `replace` parameter

currently with url.update we can add a key to the hash/query params, or update a key if it already exists. we cannot remove a key if it already exists, unless we explicitly call it with a del parameter, which only deletes. Thus we should have a replace parameter which can

• add a key if it currently doesn't exist
• remove a key if it exists in hash/search but not in the update object
• update a key if it exists in both hash/search and update object with different values

?a=1&b=1.update({b:2}, 'replace') -> ?b=2

places this would be useful eg - if we have a formhandler url in a vega spec or something similar, and want to update it on hashchange to the new hash, we currently have to hardcode a version of the url in code and update it on hashchange.