Check for id3 compilation tag (credit @zmatsuo)

Ref. pr #127, issue #83 and maybe issue #63

src/db.h

@@ -130,7 +130,7 @@ struct media_file_info {
   uint32_t item_kind;    /* song or movie */
   uint32_t data_kind;    /* dmap.datakind (asdk) */
   uint64_t sample_count;
-  char compilation;
+  uint32_t compilation;
   char artwork;
 
   /* iTunes 5+ */

src/filescanner_ffmpeg.c

@@ -118,6 +118,7 @@ static const struct metadata_map md_map_generic[] =
     { "title-sort",   0, mfi_offsetof(title_sort),         NULL },
     { "artist-sort",  0, mfi_offsetof(artist_sort),        NULL },
     { "album-sort",   0, mfi_offsetof(album_sort),         NULL },
+    { "compilation",  1, mfi_offsetof(compilation),        NULL },
 
     { NULL,           0, 0,                                NULL }
   };

src/spotify.c

@@ -476,7 +476,7 @@ spotify_metadata_get(sp_track *track, struct media_file_info *mfi, char *pltitle
   sp_artist *artist;
   sp_albumtype albumtype;
   bool starred;
-  char compilation;
+  int compilation;
   char *albumname;
 
   spotify_cfg = cfg_getsec(cfg, "spotify");