Implement currency formatting

- [x] Implementation
- [x] Tests
- [x] Doc

Fix #238 
Yeap, it's equivalent. Although, I'm not convinced the difference in performance and overall bundle size justifies the change.

> And it looks to me like `symbol` is used just once, so that could be inlined, that is, replaced with `options.style`.

Not the case in the up-to-date code.  :)