Adds optional locale to date filter

The date filter accepts a second, optional, parameter: a locale given
as a 2-letter ISO 639 language code like "en", "fr", "de". The date
gets formatted with this locale applied. Relevant for month names
(format "MMMM"). Examle: {{f|date:"MMMM":fr}}
Merge pull request #62 from dl1ely/datelocale_#61

Adds optional locale to date filter
Update project.clj
Adds example for locale usage in date filter
Merge pull request #63 from dl1ely/datelocale_#61

Adds example for locale usage in date filter
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