Allow Collator to get collation from option

Collator already have "co" in  [[RelevantExtensionKeys]] internal slot and therefore the  usage by using "-u-co-$collationkey" is already supported in ECMA402.

Close #380

spec/collator.html

@@ -35,6 +35,10 @@ <h1>InitializeCollator ( _collator_, _locales_, _options_ )</h1>
         1. Set _opt_.[[kn]] to _numeric_.
         1. Let _caseFirst_ be ? GetOption(_options_, *"caseFirst"*, *"string"*, &laquo; *"upper"*, *"lower"*, *"false"* &raquo;, *undefined*).
         1. Set _opt_.[[kf]] to _caseFirst_.
+        1. Let _collation_ be ? GetOption(_options_, `"collation"`, `"string"`, *undefined*, *undefined*).
+        1. If _collation_ is not *undefined*, then
+          1. If _collation_ does not match the Unicode Locale Identifier `type` nonterminal, throw a *RangeError* exception.
+        1. Set _opt_.[[co]] to _collation_.
         1. Let _relevantExtensionKeys_ be %Collator%.[[RelevantExtensionKeys]].
         1. Let _r_ be ResolveLocale(%Collator%.[[AvailableLocales]], _requestedLocales_, _opt_, _relevantExtensionKeys_, _localeData_).
         1. Set _collator_.[[Locale]] to _r_.[[locale]].