Allow customizing date format in DateNav

See https://github.com/concrete5japan/concrete5/commit/e8e6657f854571a7dad582531b630562510b9ffd#diff-9d04cbbed56e853341a0a40f2dd30ff5R58
Merge pull request #1626 from mlocati/language-dependant-datenav-view-date

Allow customizing date format in DateNav