findOutgoingLinks method should return only URLs in <a> tags

[aaronpk]

Currently the findOutgoingLinks method will return any mf2 property that is a URL, regardless of the element the value came from. For example, <data class="u-in-reply-to" value="http://example.com/"> will be considered a link, even though according to HTML it is not a link. This is a problem because sending a webmention to that URL will fail when its webmention endpoint tries to look for a link back.