[googleplus] Fix upload date extraction
Now --all-sub is a modifier to --write-sub and --write-auto-sub (closes #1412)

For keeping backwards compatibility --all-sub sets --write-sub if --write-auto-sub is not given
[mixcloud] Rewrite extractor (fixes #278)
Set the ext field for each format
[soundcloud] Add an extractor for users (closes #1426)