Add Mongolian flag

Adding a famfamfam icon would introduce a new flag "style" into the mix, so I'm afraid I can't accept such submissions at the moment.

I did however go to Wikipedia and fetch their flag (which also has the advantage of being Public Domain, so we won't get into a GPL conflict), convert it to a PNG, and then committed that to 7.x-1.x and 6.x-2.x - so there will be a Mongolian flag icon in future releases of Language icons. :)