Enable remaining Comrak GFM extensions
Enable remaining Comrak GFM extensions

Fixes #983

steveklabnik/crates.io,steveklabnik/crates.io,steveklabnik/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io