Allow contradictory options in various config files

[phihag]

More specific channels (System config < user config < command-line) should override less specific ones.

The parsing process should go like

default_settings = {'template': '%(id)s.%(ext)s'}
settings = default_settings.copy()
args = readArgs('/etc/youtube-dl.conf')
settings.update(parseArgs(args, errmsg='invalid system configuration'))
args = readArgs('~/.config/youtube-dl.conf')
settings.update(parseArgs(args, errmsg='invalid user configuration'))
settings.update(parseArgs(sys.argv))