Add Khmer support request

Hello. Thank you for great plugin!

I found Khmer language code definition difference between Bogo and language pack.
https://gist.github.com/mayukojpn/5070dc5576d5fdcb87716b1aac74406b

This code is working on my live blog and you can see Khmer flag on a language switcher:
http://blog.mayuko.me

Thank you!