[core] Add a mustache function to translate markdown into html

[aalmiray]

Useful for changelog, announcer, and packager templates.

Should turn this

{{#f_md2html}}
This is **bold**
This is _italic_
A list follows:

* item 1
* item 2

And now [link]({{projectLinkHomepage}})
{{/f_md2html}}

into this

<p>This is <strong>bold</strong>
This is <em>italic</em>
A list follows:</p>
<ul>
<li>item 1</li>
<li>item 2</li>
</ul>
<p>And now <a href="https://acme.com/app">link</a></p>

commonmark-java has no external dependencies.