Use URLSearchParams for paramsObject

[cailyncodes]

Currently, the paramsObject in iron-query-params.html is a native JS object. The most noticeable limitation of this approach is that every key can have at most one value. It is impossible to have the following paramsObject:

{
  "key": "value",
  "key": "otherValue"
}

The ability to have multiple values for one key is a fairly common need, especially in faceted searching.

The WHATWG spec defines a URLSearchParams class for interacting with query parameters of a URL. Web developers will be able to use a standard for interacting with query parameters of the URL instead of the custom implementation noted above.

This is a breaking change, but I feel there is great value in using a native class for query parameters. It will make the use and interaction with query parameters much more rich, as well as providing support for otherwise unattainable use cases.