Create a shortcode to embed html in markdown content files

As discussed in eclipsefdn/it/websites/eclipse.org#792 (closed), we should add a shortcode that embeds HTML onto a markdown content file.

E.g.

### Some Section Heading

Some markdown text...

{{< html >}}
  <div class="card">
    <div class="card-header">
      <h4>Some Heading</h4>
    </div>
    <div class="card-body">
      <!-- Some text -->
    </div>
    <div class="card-footer">
      <a class="btn btn-primary" href="#">Next</a>
    </div>
  </div>
{{</ html >}}