This PR adds a (killer? 🤩) functionality, namely the ability to consume any Web site / HTML source, also when an RSS / Atom feed is not available.

It is based on XPath 1.0, which is natively available in PHP:

FreshRSS can then also republish as RSS.

This can be combined with our existing ability to follow article links to retrieve full article content (use with parsimony).

Contributes indirectly to other issues: