Plug.Conn.Query.encode, creates query strings.

Here's the `encode` complement to the already-implemented `decode` function.

I would have liked to use `URI.encode_query` on the final list of field/value tuples, but that encoding function encodes the `[` and `]` characters, which makes it pretty unreadable, so I just did my own escapes and joins with `&` and `=`.

Always open to suggestions and criticism! :)
Oh, I did it that way so it'd be easy to plug the result directly into `URI.encode_query` if it changed in the future to not escape `[` and `]`. I can do the string creation right there if you'd prefer. :)