0.7.4

This release makes significant changes to elfeed-score:

• elfeed-score.el had grown to over 2K LoC, making it difficult to read, maintain & reason about. This commit re-factors it into a number of smaller files
• the README had become bloated because it tried to be an introduction, a "getting started", and user documentation. This commit factors the user documentation out into a Texinfo manual so it will be available as part of your Info system. In addition, I've started hosting the user docs in HTML format at my personal site.
• as rules accumulated more attributes, serializing them to flat lists became untenable; this release introduces a new score file format in which they are serialized to property lists
  This will all hopefully make it easier to add some new features that have been requested & that I myself have in mind.