Adds optional locale to currency-format filter

currency-format filter takes an optional parameter (ISO 639 2-letter
language code) as locale to use for rendering currencies. Default is "en".
Adds an optional locale to number-format filter

An optional ISO 630 2-letter language code can be appended to the
number-format filter to use that locale for formatting.
Unifies let form for date, currency-format und number-format

Uses locale in let form the same way for the three filters
Merge pull request #64 from dl1ely/currencynumberlocale

Adds optional locales to currency-format and number-format