Add a way to change the extension language independently of Twitch

[jleclanche]

Twitch users currently do not realize they have to change the Twitch site language in order to change the card language. This is not explained anywhere and isn't intuitive at all.
Furthermore, it's a common configuration for a viewer to have their UI in a non-english language but still be used to Hearthstone in English and there is no way to support this at this time.

Adding a language selector to allow viewers to change the extension's language fixes this.

This story is complete once:

• A language selector is available somewhere obvious-but-nonintrusive in the UI (eg. in the Deck titlebar)
• The language selector allows viewers to update the extension's Hearthstone language on a semi-permanent basis (eg. in a cookie) to any of the supported languages
• A Twitch extension release has been made with the added functionality