Normative: Add calendar and numberingSystem options

This patch allows calendar and numberingSystem to be specified in
the options bag of the DateTimeFormat and NumberFormat constructors.
One use case for these options would be, when working with locales
which have two numbering systems and calendars in use--the UA default
may be the non-Western one, but in some contexts, it may be appropriate
to use the Western one. Currently, without the patch, it would be
necessary to parse the BCP 47 language tag in the application, but Intl
provides no library to do so. For example, this all occurs in this bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1370086

Related bug: tc39#105 . This patch leaves out "collation" because of a lack
of clear use cases.

spec/datetimeformat.html

@@ -81,6 +81,12 @@ <h1>InitializeDateTimeFormat ( _dateTimeFormat_, _locales_, _options_ )</h1>
         1. Let _opt_ be a new Record.
         1. Let _matcher_ be ? GetOption(_options_, *"localeMatcher"*, *"string"*, &laquo; *"lookup"*, *"best fit"* &raquo;, *"best fit"*).
         1. Set _opt_.[[localeMatcher]] to _matcher_.
+        1. Let _calendar_ be ? Get(_options_, *"calendar"*).
+        1. If _calendar_ is not *undefined*, set _calendar_ to ? ToString(_calendar_).
+        1. Set _opt_.[[ca]] to _calendar_.
+        1. Let _numberingSystem_ be ? Get(_options_, *"numberingSystem"*).
+        1. If _numberingSystem_ is not *undefined*, set _numberingSystem_ to ? ToString(_numeringSystem_).
+        1. Set _opt_.[[nu]] to _numeringSystem_.
         1. Let _hc_ be ? GetOption(_options_, *"hourCycle"*, *"string"*, &laquo; *"h11"*, *"h12"*, *"h23"*, *"h24"* &raquo;, *undefined*).
         1. Set _opt_.[[hc]] to _hc_.
         1. Let _localeData_ be %DateTimeFormat%.[[LocaleData]].

spec/numberformat.html

@@ -55,6 +55,9 @@ <h1>InitializeNumberFormat ( _numberFormat_, _locales_, _options_ )</h1>
         1. Let _opt_ be a new Record.
         1. Let _matcher_ be ? GetOption(_options_, *"localeMatcher"*, *"string"*, &laquo; *"lookup"*, *"best fit"* &raquo;, *"best fit"*).
         1. Set _opt_.[[localeMatcher]] to _matcher_.
+        1. Let _numberingSystem_ be ? Get(_options_, *"numberingSystem"*).
+        1. If _numberingSystem_ is not *undefined*, set _numberingSystem_ to ? ToString(_numeringSystem_).
+        1. Set _opt_.[[nu]] to _numeringSystem_.
         1. Let _localeData_ be %NumberFormat%.[[LocaleData]].
         1. Let _r_ be ResolveLocale(%NumberFormat%.[[AvailableLocales]], _requestedLocales_, _opt_, %NumberFormat%.[[RelevantExtensionKeys]], _localeData_).
         1. Set _numberFormat_.[[Locale]] to _r_.[[locale]].