Display date of update in post header

[s4b7r]

I looked for a way to display the 'update' date of an edited post. Didn't take too long to implement it. If you like, you can pull this upstream.

[skyn1x0]

<p class="post-meta">
    {% if page.date-updated %}
    <time datetime="{{ page.date }}">Published {{ page.date | date: site.theme_config.date_format }}</time>
    <br><time datetime="{{ page.date-updated }}">Updated {{ page.date-updated | date: site.theme_config.date_format }}</time>
    {% else %}
    <time datetime="{{ page.date }}">{{ page.date | date: site.theme_config.date_format }}</time>
    {% endif %}
  </p>

I thought it looked a bit weird having text beside the updated date and no text beside the published date, so here's a change for that. Might just be my personal preference, but figured it might be useful to someone else.