Add support for chrono

[failable]

error[E0277]: the trait bound `DateTime<Utc>: Valuable` is not satisfied
  --> src/dolphin-bot/src/types.rs:10:20
   |
6  | #[derive(Debug, Clone, Serialize, Deserialize, Valuable)]
   |                                                -------- required by a bound introduced by this call
...
10 |     pub timestamp: Option<DateTime<Utc>>,
   |                    ^^^^^^^^^^^^^^^^^^^^^ the trait `Valuable` is not implemented for `DateTime<Utc>`
   |
   = help: the following other types implement trait `Valuable`:
             &T
             &[T]
             &mut T
             &std::path::Path
             &str
             ()
             (T0, T1)
             (T0, T1, T2)
           and 79 others
   = note: required for `std::option::Option<DateTime<Utc>>` to implement `Valuable`