Add basic --download-archive option

Often, users want to be able to download only videos they haven't seen before, despite the video files having been deleted or moved in the mean time.
When --download-archive FILE is given, the extractor and ID of every download is recorded in the specified file. If it is already present, the video in question is skipped.
Remove superfluous parenthesis
Merge branch 'download-archive'

Conflicts:
	youtube_dl/YoutubeDL.py
	youtube_dl/__init__.py