Add Credit property to Image rule

[pestevez]

This PR adds the Credit property to the Image rule.

Test Plan

Generate an image with caption and credits.

Example

• URL: http://www.civilbeat.org/2018/01/why-hawaiis-unlicensed-elder-care-industry-is-out-of-control/
• Image Rule:
  • Selector: div.cb-image
  • URL property: img
  • Attribute: src
  • Caption property: p.wp-caption-text
  • Attribute: content
  • Credit property: p.wp-caption-source
  • Attribute: content

Expected Result

The generated Image element should have the following structure:

<figure>
  <img .../>
  <figcaption>
    <cite>...<cite/>
  </figcation>
</figure>

[mburak]

LGTM. Thanks @pestevez!

[pestevez]

Thanks, @mburak!