Add "nu" as RelevantExtensionKeys

[FrankYFTang]

@anba @zbraniecki @mathiasbynens @Ms2ger @littledan @gsathya
Follow up #96

[littledan]

Thanks for this patch. Merging per previous consensus.

[anba]

• Does UTS 35 explicitly mention "nu" for relative time formatting or is it implicitly used, cf. 12.3.3 Internal slots:

[...] and implicitly "nu" for the numbering system of the number format used for numbers within the date format.

• Shouldn't Intl.RelativeTimeFormat.prototype.resolvedOptions be updated to reflect the numbering system option in the output object?

[FrankYFTang]

• Does UTS 35 explicitly mention "nu" for relative time formatting or is it implicitly used, cf. 12.3.3 Internal slots:

[...] and implicitly "nu" for the numbering system of the number format used for numbers within the date format.

Good question. Probably I should change the wording from
"Unicode Technical Standard 35 describes one locale extension key that is relevant to relative time formatting, "nu" for numbering system."
to
"Unicode Technical Standard 35 implicitly describes one locale extension key that is relevant to relative time formatting, "nu" for numbering system."
But where is it in the UTS35 which "describes three locale extension keys that are relevant to date and time formatting, "ca" for calendar, "tz" for time zone, "hc" for hour cycle" ? I have hard time to figure out WHERE that is in UTS35 so I can check is "nu" there or not.

[FrankYFTang]

• Shouldn't Intl.RelativeTimeFormat.prototype.resolvedOptions be updated to reflect the numbering system option in the output object?

Make sense, I will follow up with a PR for both issues above.