command: export mpv configure arguments as property

It seems strange that a client API user can't get this string, other
than analyzing the mpv log output.
command: fix weird stuff

Happens to fix some aspects of some properties (especially reading
"option-flags/x" as native value).
command: make sub-properties more flexible

This makes it work with all kind of types, instead of just some simple
ones.
command: rename "option-flags" property to "option-info"
command: export some option metadata

This might be interesting for GUIs and such.

It's probably still a little bit insufficient. For example, the filter
and audio/video output lists are not available through this.