Use a Spotify song's title instead of a YouTube video's title (#99)

* Use a Spotify song's title instead of a YouTube video's title

* Added fallback to YouTube title if song's metadata cannot be fetched from Spotify

* Removed duplicate generation of metadata

* Fix test cases that use download_song(..) and generate_filename(..)

* Fix conflicting function

* Fix conflict in check_exists()

* Fix filenames for non spotify songs

* Fix some bugs

* Some documentation changes

* Remove unnecessary determine_filename()

core/misc.py

@@ -77,13 +77,13 @@ def is_spotify(raw_song):
         return False
 
 
-def generate_filename(title):
+def sanitize_title(title):
     """Generate filename of the song to be downloaded."""
     title = title.replace(' ', '_')
 
     # slugify removes any special characters
-    filename = slugify(title, ok='-_()[]{}', lower=False)
-    return filename
+    title = slugify(title, ok='-_()[]{}', lower=False)
+    return title
 
 
 def generate_token():