Allow specification of abbreviation in thousands, millions, billions, or...

... trillions

Augmenting the existing syntax of 'a' for abbreviation, the format can additionally specify
  'aK' for thousands,
  'aM' for millions,
  'aB' for billions,
  'aT' for trillions

e.g. `numeral(-3222111).format('0,0 aK')` --> `-3,222 K`