Adding support for single files (not only directories)

[Muffo]

However but I think the app should detect when a single file is passed as argument and change the behavior accordingly.

At the moment if I try to download the subtitles of a single file I get this error:

python lm.py MyFile.mkv --download eng
Traceback (most recent call last):
  File "../lm-master/lm.py", line 1514, in <module>
    LM.download_subtitle(files, options.download)
  File "../lm-master/lm.py", line 1073, in download_subtitle
    ref, query = self.download_subtitles_query( files, language )
  File "../lm-master/lm.py", line 1116, in download_subtitles_query
    old_subs = [ old for old in filelist(filedir,False) \
  File "../lm-master/lm.py", line 97, in filelist
    for f in os.listdir(dir):
OSError: [Errno 2] No such file or directory: ''

Having the support for a complete directory can be very convenient.