add sticker type (#653)

* add sticker type

this pr adds the message type 'sticker'.
stickers are handled as normal images
but tagged with the header `Chat-Content: sticker`
it's up to the ui to render these stickers appropriate.

* cargo fmt

src/mimefactory.rs

@@ -435,6 +435,10 @@ impl<'a> MimeFactory<'a> {
                     }
                 }
 
+                if self.msg.type_0 == Viewtype::Sticker {
+                    wrapmime::new_custom_field(imf_fields, "Chat-Content", "sticker");
+                }
+
                 if self.msg.type_0 == Viewtype::Voice
                     || self.msg.type_0 == Viewtype::Audio
                     || self.msg.type_0 == Viewtype::Video