add "category" tag to parser

[glennj]

From https://tools.ietf.org/html/rfc4287#page-18, the Atom "category" tag has a "term" element with the text of the category.

As an example in the wild, the Environment Canada weather feeds make heavy use of the category: <link rel="self" href="https://www.weather.gc.ca/rss/city/on-118_e.xml" type="application/atom+xml"/>