Add date template support to itinerary component

[mihaibirsan]

This pull request adds support for including formatted dates in the itinerary component. Users can now use the {{date}} or {{date:YYYY-MM-DD}} template tags to include dates in the itinerary. The available date formats can be found here.

Here's a simple example for adding a date header to a separate itinerary page:

And here's my own setup with daily one-liners about the year, week and current day: