Update pulldown-cmark dependency to 0.12

Author: dtolnay

Cargo.toml

@@ -21,7 +21,7 @@ num_cpus = "1.0"
 oqueue = "0.1"
 parking_lot = "0.12"
 pin-project = "1.0"
-pulldown-cmark = "0.11"
+pulldown-cmark = "0.12"
 rayon = "1.0"
 regex = "1.0"
 remain = "0.2"