Simple cmark-gfm Rust wrapper

This crates has been developed to cover only the main functions of cmark-gfm. Feel free to propose PR!

Example

```rust extern crate cmark_gfm;

use cmark_gfm::{Options, Parser, Render};

fn main() { let text = &"# Hello world";

let mut options = Options::empty();
options.insert(Options::CMARK_OPT_FOOTNOTES);

let parser = Parser::new(options);

let extensions = &["table", "strikethrough", "autolink", "tasklist"];
for extension in extensions {
    parser.add_extension(extension).unwrap();
}

parser.parse(text);
let result = Render::to_html(&parser);

println!("Output : {}", result);

} ```

License

MIT