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