Exception on leading and trailing whitespaces in custom schemes

[Badgie]

If there are leading or trailing whitespaces in input on custom schemes, these will be caused the empty string to be included in the list of seeds - this will cause an exception.

$ spotirec -ac -l 100 --tune min_tempo=160
'0 1 9 31 '
Traceback (most recent call last):
  File "/usr/bin/spotirec", line 727, in <module>
    parse()
  File "/usr/bin/spotirec", line 687, in parse
    print_artists_or_tracks(api.get_top_list('artists', 50, headers=headers))
  File "/usr/bin/spotirec", line 214, in print_artists_or_tracks
    parse_seed_info([data['items'][int(x)] for x in selection.split(' ')])
  File "/usr/bin/spotirec", line 214, in <listcomp>
    parse_seed_info([data['items'][int(x)] for x in selection.split(' ')])
ValueError: invalid literal for int() with base 10: ''