Move regex to a class.

Signed-off-by: cyberzhg <cyberzhg@gmail.com>
Convert Markdown to HTML.
Add: atx header, setext header, horizontal, inline link, emphasis, strong.

Signed-off-by: cyberzhg <cyberzhg@gmail.com>
Previewer could work.

Signed-off-by: cyberzhg <cyberzhg@gmail.com>
Merge branch 'html'