Parser should ignore rel values from HTML tags that don't allow rel values

[aaronpk]

Example HTML:

<b href="http://example.com/" rel="test">test</b>

Parsed result: http://pin13.net/mf2/?id=20160414153037030

The parser finds the rel value and adds the href value to the list of rels. However the <b> tag is not a hyperlink, and the href and rel attributes have no meaning defined.

The parser should only use rel values on a, link, and area tags according to the HTML spec. https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes

[aaronpk]

Fixed in #92