Implement currency formatting

- [x] Implementation
- [x] Tests
- [x] Doc

Fix #238 
Right, that's what the `if` checks after all. With that in mind, maybe change `currencySymbolPattern` to also return an object and change the check? Or just change the check to use `options.style === "symbol"`, that would also be more explicit. Either way, my question should make it clear that the current implementation is rather difficult to grasp.