Using os.path to determine file basename

Bugfix.

When the video file is not present in the directory
the subtitle was downloaded with the same name provided in input.
This commit redefines 'remove_extension' and fix this.
Added code to handle partial downloads (.part and qBittorrent)
Merge branch 'sub_extension_fix' of github.com:lucacerone/addic7ed-cli into sub_extension_fix