config: avoid code duplication
config: add 'replaygain' option
README: simplify the example configuration

Users can read the manpage if they want.
docs: users shouldn't use 'filter'

It exposes too much of mpv internals. Undocument it for now (like 'script')
until I find a better solution.
config: minor clean-up

Sort options alphabetically.
config: add 'notify' option
CMakeLists.txt: properly install scripts
player: rework state machine and random track mechanism

a) Merge IDLE status into STOPPED.
b) Use the "append-play" mode of loadfile instead of using loadfile +
   set_property(playlist-pos, ...).
c) Don't manually track the playlist position.
d) Misc. clean-up.

Note that this requires mpv v0.7 because of the changes to append-play.