feat: let composes carry custom frontmatter headers

src/demo.rs

@@ -227,6 +227,7 @@ fn plant(dir: &Path) -> Result<(), String> {
             to: "grace@example.org".into(),
             subject: "Re: quarterly infrastructure review".into(),
             body: "started a reply, will finish it after the review call".into(),
+            headers: vec!["Urgency: low".into()],
             reply_to: None,
             at: now() - 3 * 3600,
         }],