support metadata extraction from HTML tables

[tripodsan]

with adobe/helix-markdown-support#27.

the metadata table will look like:

<table>
  <tr>
    <td><p>Metadata</p></td>
  </tr>
  <tr>
    <td><p>Title</p></td>
    <td><p>Pricing | Adobe Spark</p></td>
  </tr>
  <tr>
    <td><p>Description</p></td>
    <td><p>Adobe Spark lets you easily search from thousands of free photos, use themes, add filters, pick fonts, add text to photos, and make videos on mobile and web.</p></td>
  </tr>
  <tr>
    <td><p>Image</p></td>
    <td><p><img src="https://hlx.blob.core.windows.net/external/1d9b5338b3228f77275f5e878c29e1c21822ef706#image.png" alt=""></p></td>
  </tr>
</table>