Add support for kCTLanguageAttributeName

[ZevEisenberg]

In WWDC 2013 session 223, at around 39:20, Ned Holbrook introduces language tagging with the kCTLanguageAttributeName attribute. This is distinct from the accessibility language tag which we added support for in #155. I don't know whether it also affects how VoiceOver reads the string, but the talk mostly mentions it in the context of using language-sensitive variants of glyph substitution, line breaking, and font fallbacks. It can currently be used via extraAttributes, but it would be nice to have first-class support for it.