Expose escaping/unescaping html as an option/utility

[wheresrhys]

I'm using markdown as the authoring syntax for a cli tool - I'm able to hook in to the renderer to replace html tags with calls to chalk.

The one place I'm not able to decouple markdown from html is escaped characters being converted into html entities. Would you accept a PR to enable turning off html escaping via an option?

[adam-lynch]

Duplicate of #529

[mharward]

This would be excellent and a problem we have also run into (in a slightly different context).

[amasad]

I've worked around this by using lodash's unescape

const renderer = new marked.Renderer();

// Pass html unescaped
renderer.html = str => _.unescape(str);

[joshbruce]

#985