Add ColorType::Multiband enum variant to support samples>1 per pixel (#224)

Co-authored-by: tromper <scott@hopkinsunderground.com>

src/lib.rs

@@ -40,4 +40,7 @@ pub enum ColorType {
 
     /// Pixel is YCbCr
     YCbCr(u8),
+
+    /// Pixel has multiple bands/channels
+    Multiband { bit_depth: u8, num_samples: u16 },
 }