The most important functionality of this class is formatting numbers. This is
an implementation of Number Format Patterns as defined in Unicode Technical
Standard #35. However, it's not complete implementation as for now.
Following features are missing (in brackets - chapter from specification):
- support for escaping of special characters in format string [part of G.2]
- formatting numbers to scientific notation [G.4]
- support for significant digits [G.5]
- support for padding [G.6]
- formatting numbers in other number systems than "latn"
- currency symbol substitution is simplified