Add blockquote tag

[JackMorganNZ]

The standard Markdown blockquote tag is limited in formatting (can be improved by CSS), but it would be nice to have a Verto tag to allow finer editing, especially for automatic use with Bootstrap 4. Possibly could look like:

{blockquote}

First and foremost, we believe that speed is more than a feature.

- [Fred Wilson](https://en.wikipedia.org/wiki/Fred_Wilson_(financier))

{blockquote end}

This could be used with a template like the following:

<blockquote class="blockquote">
  {{ content }}
  <footer class="blockquote-footer">
    {{ source_content}}
  </footer>
</blockquote>

to produce something like the following

<blockquote class="blockquote">
  <p>First and foremost, we believe that speed is more than a feature.</p>
  <footer class="blockquote-footer">
    [Fred Wilson](https://en.wikipedia.org/wiki/Fred_Wilson_(financier))
  </footer>
</blockquote>

Would need to figure out how to detect footer information (could be argument value like image alt text, though it wouldn't work for in-context translation (though this may be a worth compromise for a simpler tag).

Optional arguments

• align - Text value to be passed through to template.
• source - URL text value to be passed through to the template.