Added a playlist download option.

Summary:
    - Speeding up by using go feature : concurrency.
    - Prompting for Yes/No.
TODO :
    - One single bar for the download pool.

pkg/soundcloud/playlist.go

@@ -0,0 +1,60 @@
+// get list of DownloadTrack of all the urls in the playlist
+// with the number of urls in the playlist
+
+package soundcloud
+
+import (
+	"bytes"
+	"encoding/json"
+	"log"
+	"net/http"
+	"strconv"
+
+	"github.com/AYehia0/soundcloud-dl/pkg/client"
+)
+
+type Id struct {
+	Id int `json:"id"`
+}
+
+type Tracktmp struct {
+	Tracks []Id `json:"tracks"`
+}
+
+// get all the urls inside the playlist
+func GetPlaylistTracks(track *SoundData, clientId string) []SoundData {
+	ids := make([]string, 0)
+	trackIds := Tracktmp{}
+	plApiUrl := GetTrackInfoAPIUrl(track.PermalinkUrl, clientId)
+
+	statusCode, data, err := client.Get(plApiUrl)
+
+	if err != nil || statusCode != http.StatusOK {
+		return nil
+	}
+
+	//fmt.Println(string(data))
+	json.Unmarshal(data, &trackIds)
+
+	for _, t := range trackIds.Tracks {
+		ids = append(ids, strconv.Itoa(t.Id))
+	}
+
+	tApiUrl := GetTracksByIdsApiUrl(ids, clientId)
+
+	statusCode, data, err = client.Get(tApiUrl)
+
+	if err != nil || statusCode != http.StatusOK {
+		return nil
+	}
+
+	sounds := make([]SoundData, 0)
+
+	dec := json.NewDecoder(bytes.NewReader(data))
+	if err := dec.Decode(&sounds); err != nil {
+		log.Println("Error decoding json: ", err)
+		return nil
+	}
+
+	return sounds
+}