command: improve A-B loop behavior

If the B point is set, then loop back to A.

Also, update the OSD bar if necessary.
command: add an ab_loop command

As suggested in #1241; to make using the feature easier.

Also add better OSD-formatting for the ab-loop-a/b properties.
manpage: document vo_cmdline command
demux_mkv: add an option for compatibility with Haali

This was requested on IRC.
manpage: fix typo
lua: add youtube-dl hook script

This merely adds the file without using it, for the sake of retaining
authorship information.

Signed-off-by: wm4 <wm4@nowhere>
player: integrate ytdl_hook.lua
input.conf: add some additional bindings
manpage: changes.rst: add youtube-dl script
options: add --ytdl-format option for youtube-dl format

It's passed with the '--format' option to youtube-dl.

If it isn't set, we don't pass '--format best' so that youtube-dl can
use the options from its configuration file.

Signed-off-by: wm4 <wm4@nowhere>
vd_lavc: flush frames before uninitializing hw decoder

This way, no surfaces are in use when uninitializing the hw decoders,
which might help with -copy hw decoders (normal hw decoding is not
affected).