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Add the overwrite true|false option.

This option, if set to true, will overwrite existing downloaded files
but if set to false, will skip downloads for which there are existing
files in the current download directory. This allows for quickly
downloading new files in a playlist while ignoring the ones that are
already downloaded.
adding support for notifiers
removing unnecessary import
Merge remote-tracking branch 'origin/pr/93' into develop
Fix order of help set topics
use subprocess.call for notifier, don't depend on mplayer
fix notifier bug with shell=True
Merge remote-tracking branch 'origin/pr/95' into develop-pr95
fix notify command doesn't work with ' char
remove unnecessary print
Exit with correct status code