Parser: add "title" field and set it with the first heading found

src/message/parser.rs

@@ -112,6 +112,7 @@ impl<'a> GemtextNode {
 #[derive(Debug)]
 pub struct Parser<'a> {
     state: State<'a>,
+    title: Option<String>,
     lines: Vec<GemtextNode>,
 }
 
@@ -119,6 +120,7 @@ impl<'a> Parser<'a> {
     pub fn new() -> Self {
         Self {
             state: State::Normal,
+            title: None,
             lines: vec![]
         }
     }
@@ -139,7 +141,14 @@ impl<'a> Parser<'a> {
     }
 
     fn heading(&mut self, line: &'a str) {
-        self.lines.push(GemtextNode::parse_heading(line));
+        let line = GemtextNode::parse_heading(line);
+        if self.title.is_none() {
+            match &line {
+                GemtextNode::Heading1(t) => self.title = Some(t.clone()),
+                _ => {},
+            }
+        }
+        self.lines.push(line);
     }
 
     fn list_item(&mut self, line: &'a str) {