Move Date object from `page.title` to `page.date`

Moves the Date object from `page.title` to `page.date`:
- In case of a non-date archive, `page.title` is the name of the category or tag, and `page.date` is `nil`
- In case of a date archive, `page.title` is `nil` and `page.date` is the Date object previously stored in `page.title`

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- [x] Implement code
- [ ] Add tests
- [ ] Update documenation

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Addresses #20.