Add HTML input support #312
Description
For HTML processing as in https://github.com/programmieraffe/oerhoernchen20 (with sources configured as in https://github.com/programmieraffe/oerhoernchen20/blob/master/scrapy/projects.json), we need an HtmlReader, to be used between HttpOpener and XmlDecoder (parsing HTML to XML, e.g. with https://jsoup.org/) in a workflow to create JSON index data for https://github.com/orgs/hbz/projects/4.