post: Make url optional and add optional alt

r621/src/post.rs

@@ -89,7 +89,8 @@ pub struct PostSample {
     pub has: bool,
     pub width: ImageDimension,
     pub height: ImageDimension,
-    pub url: String
+    pub url: Option<String>,
+    pub alt: Option<String>
 }
 
 #[derive(serde::Serialize, serde::Deserialize, Clone, Debug)]