Handle atom feed with space around CDATA

Trim space around CDATA elements before extracting the CharData.

This problem was discovered when reading https://www.sethvargo.com/feed.xml.
Title and Summary fields have newlines and space between the <title>
element and the CDATA element. e.g.

  <title>
    <![CDATA[Entry title here]]>
  </title>

This meant the title of the feed was coming into MiniFlux as,
  <![CDATA[Entry title here]]>