Add Likely Subtags first step

[Caoimhin O Donnaile]

If the tags were only being used to tag written language, I think people 
would assume that en-oxford meant the spelling and standards adopted by 
the Oxford English Dictionary (en-GB but with -ize rather than -ise).

But if the tags were being used to tag spoken language, I think people 
would assume it meant Oxford/Oxbridge English, a sort of educated, 
public-school, semi-posh accent:

   https://www.youtube.com/watch?v=zg1AQ5JmU9A (at 3:08)
   http://www.bbc.co.uk/voices/yourvoice/feature2_4.shtml

or (less likely) the very different dialect of rural Oxfordshire.

I see that the Wikipedia article on Received Pronunciation hedges a bit 
between spoken and written:

  “The study of RP is concerned exclusively with pronunciation, whereas
   "Standard English", "the Queen's English", "Oxford English", or "BBC
   English" is also concerned with matters such as grammar, vocabulary and
   style.”

Caoimhín