Expose chrono API

[snylonue]

So far, this crate uses chrono's [NaiveDate](https://docs.rs/chrono/0.4.19/chrono/naive/struct.NaiveDate.html) to represent date.

However, chrono implements some apis like year(), month() and day() on its trait Datelike, making it impossible to get them without explicitly importing chrono, which is the most important parts of a date.

Thus, it would be convenient to expose such trait via e.g. pub use chrono::Datalike or add getters on entity types.