Try to always decode HTML <content> and <summary>

Some feed have

    <summary type="html"><![CDATA[ ... ]]></summary>
    <content type="html"><![CDATA[ ... ]]></content>
    <content type="html"> (escaped HTML) </content>

This commit tries to decode the HTML into a structured representation.
Stop the parsing oh HTML content on serious errors

In these cases, the HTML will stay unchanged (as a single leaf).  This
allows other tools to try to decode it.
Allow to specify the encoding for HTML parsing
Merge pull request #27 from Chris00/master

Try to always decode HTML <content> and <summary>