gatsby-plugin-feed: add channel image

[cedricdelpoux]

It could be useful to add channel image to the RSS feed:

<channel>
  <title>My Site</title>
  <image>
    <url>https://mysite.com/my-image.png</url>
    <title>My Site</title>
    <link>https://www.mysite.com</link>
    <width>32</width>
    <height>32</height>
  </image> 
</channel>