feat: add cargo today command (#43)

src/template/day.rs

@@ -2,6 +2,9 @@ use std::error::Error;
 use std::fmt::Display;
 use std::str::FromStr;
 
+#[cfg(feature = "today")]
+use chrono::{Datelike, Local};
+
 /// A valid day number of advent (i.e. an integer in range 1 to 25).
 ///
 /// # Display
@@ -37,6 +40,19 @@ impl Day {
     }
 }
 
+#[cfg(feature = "today")]
+impl Day {
+    /// Returns the current day if it's between the 1st and the 25th of december, `None` otherwise.
+    pub fn today() -> Option<Self> {
+        let today = Local::now();
+        if today.month() == 12 && today.day() <= 25 {
+            Self::new(u8::try_from(today.day()).ok()?)
+        } else {
+            None
+        }
+    }
+}
+
 impl Display for Day {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{:02}", self.0)