Add `include-html` directive

[mondeja]

As Markdown is a subset of HTML, it's very common to face a situation in which Markdown needs to include HTML. The main problem trying to do so is that, contrary to Markdown, HTML is a language that doesn't depend on formatting.

This leads to really unexpected situations. For example, if you want to include an HTML snippet into a Markdown table cell, the newlines of the Markdown file must be removed or the inclusion will break the table. See jekyll/jekyll#9368 as an example in Jekyll.

So adding a include-html directive could solve that by removing characters that can be ignored in HTML in the inclusion.