Extension fallback

[ocadaruma]

I think it is useful to add extension fallback feature to SbtTwirl.
It will work as follows.

• If you have set formatFallback: SettingKey[Option[String]] and template's extension does not match any in templateFormats, then formatFallback will be used.
  (formatFallback will be Some("play.twirl.api.TxtFormat"))