Be consistent with "list" responses.

Fix links and lots of broken markup.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default.  This way, one can enable `TAG_COMMENT` without
having to list all the other tags.

Add link to more recent libsidplayfp and correct URL pointing to libsidplay2.

Closes #89

Will allow more complex fitler expression, such as negation (#89).

Was converted to reStructuredText / Sphinx in commit
8a689e68