Track matching issue . Support for sub_tracks

[jesseward]

Creating from issue #33

I ran into a track number matching issue - releases like 998336 for example are reported back as having zero tracks and hence not matched to the total number of tracks in the folder (24 in this case). It seems any track on discog with the return carriage sign is not recognised - I tried another album which had 2 tracks out of 18 marked the same and discotagger reported back 16 tracks in the album instead of 18.

example JSON
https://www.discogs.com/release/998336
https://api.discogs.com/releases/998336

  "tracklist": [
    {
      "duration": "",
      "position": "",
      "type_": "index",
      "title": "Pictures At An Exhibition",
      "sub_tracks": [
        {
          "duration": "1:42",
          "position": "1",
          "type_": "track",
          "title": "Promenade"
        },
        {
          "duration": "2:58",
          "position": "2",
          "type_": "track",
          "title": "I. Gnomus"
        },
        {
          "duration": "1:02",
          "position": "3",
          "type_": "track",
          "title": "Promenade"
        },
        {
          "duration": "5:29",
          "position": "4",
          "type_": "track",
          "title": "II. Il Vecchio Castello"
        },
        {
          "duration": "0:35",
          "position": "5",
          "type_": "track",
          "title": "Promenade"
        },
        {
          "duration": "1:06",
          "position": "6",
          "type_": "track",
          "title": "III. Les Tuileries"
        },
        {
          "duration": "4:19",
          "position": "7",
          "type_": "track",
          "title": "IV. Bydło"
        },
        {
          "duration": "1:02",
          "position": "8",
          "type_": "track",
          "title": "Promenade"
        },
        {
          "duration": "1:46",
          "position": "9",
          "type_": "track",
          "title": "V. Ballet Des Poussins Dan Leur Coque"
        },
        {
          "duration": "2:45",
          "position": "10",
          "type_": "track",
          "title": "VI. Samuel Goldenberg Und Schmuyle"
        },
        {
          "duration": "1:43",
          "position": "11",
          "type_": "track",
          "title": "Promenade"
        },
        {
          "duration": "1:22",
          "position": "12",
          "type_": "track",
          "title": "VII. Limoges - Le Marché"
        },
        {
          "duration": "2:18",
          "position": "13",
          "type_": "track",
          "title": "VIII. Catacombae: Sepulcrum Romanum"
        },
        {
          "duration": "3:54",
          "position": "14",
          "type_": "track",
          "title": "Con Mortuis In Lingua Mortua"
        },
        {
          "duration": "4:10",
          "position": "15",
          "type_": "track",
          "title": "IX. La Cabane Sur Des Pattes De Poule"
        },
        {
          "duration": "6:07",
          "position": "16",
          "type_": "track",
          "title": "X. La Porte Des Bohatyrs De Kiev (La Grande Porte De Kiev)"
        }
      ]
    },
    {
      "duration": "",
      "position": "",
      "type_": "index",
      "title": "Valses Nobles Et Sentimentales",
      "sub_tracks": [
        {
          "duration": "1:39",
          "position": "17",
          "type_": "track",
          "title": "I. Modéré - Très Franc"
        },
        {
          "duration": "3:00",
          "position": "18",
          "type_": "track",
          "title": "II. Assez Lent, Avec Une Expression Intense"
        },
        {
          "duration": "2:45",
          "position": "19",
          "type_": "track",
          "title": "III. Modéré"
        },
        {
          "duration": "1:33",
          "position": "20",
          "type_": "track",
          "title": "IV. Assez Animé"
        },
        {
          "duration": "2:03",
          "position": "21",
          "type_": "track",
          "title": "V. Presque Lent, Dans Un Sentiment Intime"
        },
        {
          "duration": "0:30",
          "position": "22",
          "type_": "track",
          "title": "VI. Vif"
        },
        {
          "duration": "3:24",
          "position": "23",
          "type_": "track",
          "title": "VII. Moins Vif"
        },
        {
          "duration": "4:36",
          "position": "24",
          "type_": "track",
          "title": "VIII. Epilogue: Lent"
        }
      ]
    }
  ],