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