Query String Parse and Stringify

Implement two functions for URL query strings.

parseQuery(query) returns an object:

• A leading ? is ignored. An empty string gives {}.
• Pairs are separated by & and empty pairs are skipped. Each pair is split at the first =; a pair with no = has the value "".
• Keys and values are percent-decoded, and + means a space.
• A key that appears once maps to a string. A key that appears several times maps to an array of its values in order.

stringifyQuery(params) returns a query string without a leading ?:

• Pairs are key=value joined by &, in the object's key order; keys and values are encoded with encodeURIComponent.
• An array value produces one pair per item (tag=a&tag=b).
• Values are converted with String; undefined and null values (and array items) are skipped.
• An empty object gives "".

Both must round-trip simple data: parseQuery(stringifyQuery(x)) equals x when every value is a non-empty string or an array of at least two strings. Input to parseQuery is always validly encoded.