add afl fuzz target

tests/afl/src/gen.rs

@@ -0,0 +1,11 @@
+use afl::fuzz;
+
+fn main() {
+    fuzz!(|data: &[u8]| {
+        if let Ok(s) = std::str::from_utf8(data) {
+            let p = jotdown::Parser::new(s);
+            let mut output = String::new();
+            jotdown::html::push(p, &mut output);
+        }
+    });
+}