Allow lookup of EU region using alternate code QUU

[cejaekl]

What are you trying to accomplish?

Some systems use the alpha-three code QUU to refer to the European Union.
This is, in a sense, valid, because ISO 3166 defines a private-use range that includes QUU.
For compatibility with such legacy systems, this PR adds the ability to look up the EU/EUE region using code QUU.

What approach did you choose and why?

In implementing this, I realized that we never actually implemented lookup by alternate code at the "country"/continent level; we only implemented it for "provinces". So, to achieve our desired lookup, we have to also allow lookup by alternate codes at the higher levels.

The impact of these changes

Before:

irb(main):002:0> Worldwide.region(code: "QUU").full_name
=> "Unknown Region"

After:

irb(main):001:0> Worldwide.region(code: "QUU").full_name
=> "European Union"

Checklist

• I have added a CHANGELOG entry for this change (or determined that it isn't needed)