Add an option to escape or strip HTML #20

Description

Raw HTML in markdown is passed through to the output untouched, and there is no option to escape or strip it.

I could deny input containing HTML in my own validation layer, but that rejects legitimate content such as List<String> written in inline code, so a built-in option would be cleaner.

Proposal

An option controlling how raw HTML is treated, matching the three modes league/commonmark offers. This should not require an HTML sanitizer package or any DOM parsing.

new Markdown(html: HtmlMode::Allow);
new Markdown(html: HtmlMode::Escape);
new Markdown(html: HtmlMode::Strip);

html: HtmlMode::Escape is just and idea, the final name, place and way to configure this should be all yours to decide.