options: add --audio-file-paths

Requested. It works like --sub-paths. This will also load audio files
from a "audio" sub directory in the config file (because the same code
as for subtitles is used, and it also had such a feature).

Fixes #2632.

options/options.c

@@ -332,6 +332,7 @@ const m_option_t mp_opts[] = {
 
     OPT_STRING_APPEND_LIST("sub-file", sub_name, M_OPT_FILE),
     OPT_PATHLIST("sub-paths", sub_paths, 0),
+    OPT_PATHLIST("audio-file-paths", audiofile_paths, 0),
     OPT_STRING("sub-codepage", sub_cp, 0),
     OPT_FLOAT("sub-delay", sub_delay, 0),
     OPT_FLOAT("sub-fps", sub_fps, 0),

options/options.h

@@ -237,6 +237,7 @@ typedef struct MPOpts {
     int field_dominance;
     char **sub_name;
     char **sub_paths;
+    char **audiofile_paths;
     int sub_auto;
     int audiofile_auto;
     int osd_bar_visible;