Download subtitles in alphabetic order when an entire directory is selected

[ptrcnull]

currently, when a directory is selected, a seemingly random order is assumed:

$ docker run --rm -it -w $PWD -v $PWD:$PWD subliminal:2.2.0-rc1 download --language en .
Not using any configuration file.
Collecting videos  [####################################]  100%   
16 videos collected / 0 video ignored / 0 error
Downloading subtitles  [------------------------------------]    0%  Suits.S06E11.1080p.BluRay.
Downloading subtitles  [##----------------------------------]    6%  00:02:46  Suits.S06E11.108
Downloading subtitles  [##----------------------------------]    6%  00:02:46  Suits.S06E09.108
Downloading subtitles  [####--------------------------------]   12%  00:02:04  Suits.S06E09.108
Downloading subtitles  [####--------------------------------]   12%  00:02:04  Suits.S06E13.108
Downloading subtitles  [######------------------------------]   18%  00:01:38  Suits.S06E13.108
Downloading subtitles  [######------------------------------]   18%  00:01:38  Suits.S06E02.108
Downloading subtitles  [#########---------------------------]   25%  00:01:31  Suits.S06E02.108
Downloading subtitles  [#########---------------------------]   25%  00:01:31  Suits.S06E04.108
Downloading subtitles  [###########-------------------------]   31%  00:01:23  Suits.S06E04.108
Downloading subtitles  [###########-------------------------]   31%  00:01:23  Suits.S06E03.108
Downloading subtitles  [#############-----------------------]   37%  00:01:15  Suits.S06E03.108
Downloading subtitles  [#############-----------------------]   37%  00:01:15  Suits.S06E07.108
Downloading subtitles  [###############---------------------]   43%  00:01:06  Suits.S06E07.108
Downloading subtitles  [###############---------------------]   43%  00:01:06  Suits.S06E15.108
Downloading subtitles  [##################------------------]   50%  00:00:54  Suits.S06E15.108
Downloading subtitles  [##################------------------]   50%  00:00:54  Suits.S06E10.108
Downloading subtitles  [####################----------------]   56%  00:00:47  Suits.S06E10.108
Downloading subtitles  [####################----------------]   56%  00:00:47  Suits.S06E06.108
Downloading subtitles  [######################--------------]   62%  00:00:41  Suits.S06E06.108
Downloading subtitles  [######################--------------]   62%  00:00:41  Suits.S06E08.108
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Downloading subtitles  [########################------------]   68%  00:00:33  Suits.S06E05.108
Downloading subtitles  [###########################---------]   75%  00:00:26  Suits.S06E05.108
Downloading subtitles  [###########################---------]   75%  00:00:26  Suits.S06E14.108
Downloading subtitles  [#############################-------]   81%  00:00:19  Suits.S06E14.108
Downloading subtitles  [#############################-------]   81%  00:00:19  Suits.S06E16.108
Downloading subtitles  [###############################-----]   87%  00:00:12  Suits.S06E16.108
Downloading subtitles  [###############################-----]   87%  00:00:12  Suits.S06E12.108
Downloading subtitles  [#################################---]   93%  00:00:06  Suits.S06E12.108
Downloading subtitles  [#################################---]   93%  00:00:06  Suits.S06E01.108
Downloading subtitles  [####################################]  100%  Suits.S06E01.1080p.BluRay.
Downloading subtitles  [####################################]  100%                                                    
Some providers have been discarded due to unexpected errors: podnapisi
Downloaded 16 subtitles

in the totally legitimate use case when the user is impatient and wants to start watching a series, but is being held back by subtitle download, a sequential download in alphabetic order would be more useful

[getzze]

Done in #1161