Add Colour::hex

(cherry picked from commit 6c94e05)

src/utils/colour.rs

@@ -190,6 +190,22 @@ impl Colour {
     /// [`g`]: #method.g
     /// [`b`]: #method.b
     pub fn tuple(&self) -> (u8, u8, u8) { (self.r(), self.g(), self.b()) }
+
+    /// Returns a hexadecimal string of this Colour.
+    ///
+    /// This is equivalent to passing the integer value through
+    /// `std::fmt::UpperHex` with 0 padding and 6 width
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use serenity::utils::Colour;
+    ///
+    /// assert_eq!(Colour::new(6573123).hex(), "644C43");
+    /// ```
+    pub fn hex(&self) -> String {
+        format!("{:06X}", self.0)
+    }
 }
 
 impl From<i32> for Colour {