Add decorator option for language links

Callback function to be called when a language link is prepared
Can be used for custom decoration.
Arguments:
(a) the $language - the language link jQuery object
(b) languageCode

The function can do any styling, changing properites etc on the passed link.
See examples/decorator.html for example usage.

This pull request address PR #158  by @eranroz  in a generic way. Also addressing similar usecases coming in ongoing projects in WMF.