input.conf: add some bindings for changing audio-delay

Requested on: https://github.com/mpv-player/mpv/commit/90ec3334174e80c16f00971886223a3afabc1aca#commitcomment-7331673

Might remove or remap them again later.

etc/input.conf

@@ -82,6 +82,8 @@
 #I show_text "${filename}"              # display filename in osd
 #z add sub-delay -0.1                   # subtract 100 ms delay from subs
 #x add sub-delay +0.1                   # add
+#ctrl++ add audio-delay 0.100           # this changes audio/video sync
+#ctrl+- add audio-delay -0.100
 #9 add volume -1
 #/ add volume -1
 #0 add volume 1