Korean translation and non-thousand based abbreviation.

Hello, @adamwdraper! This PR has two feature.

1. Add Korean language as `ko`. ( a205da8 )
2. Implement non-thousand based abbreviation. (e.g. `1만`, `56万`, see also [Japanese numerals](http://en.wikipedia.org/wiki/Japanese_numerals) or [Korean numerals](http://en.wikipedia.org/wiki/Korean_numerals))
  - It's very similar to adamwdraper#202. but choose diffrent approach to respect other codebase.
  - I still prefer adamwdraper#202 's implementation detail too. please notice me if you have plan to merge that. (I'll close this PR.)