libcmark-gfm raw bindings

--- Rust wrapper for `libcmark-gfm`, GitHub's fork of the reference parser for CommonMark. For more information on available extensions, see the [documentation](https://github.github.com/gfm/). ### Requirements As this crates build itself `libcmark-gfm` from source, **you don't need to have `libcmark-gfm` already installed**. You need `cmake` and you will need to meet [bindgen requirements](https://rust-lang.github.io/rust-bindgen/requirements.html). ### License MIT