Improve RSS channel defaults

[reed-jones]

Set the channel link and description to match siteMetadata's URL and subtitle.

<title>Blog by John Doe</title>
<description>Blog by John Doe</description>
<link>http://github.com/dylang/node-rss</link>

becomes

<title>Blog by John Doe</title>
<description>Pellentesque odio nisi, euismod in, pharetra a, ultricies in, diam. Sed arcu.</description>
<link>https://lumen.netlify.com/</link>

also removed trailing slash in URL as it resulted in a double slash in the feed, leading to a 404

<link>https://lumen.netlify.com//posts/humane-typography-in-the-digital-age/</link>

[alxshelepenok]

Thank you