Add --ignore-config option

[phihag]

There should be an option to ignore configuration files. Do something sensible if it occurs in a configuration file.

[jaimeMF]

I would suggest to use something similar to #632, but parsing first sys.argv and only parsing the config files if --ignore-config is not given:

default_settings = {'template': '%(id)s.%(ext)s'}
settings = default_settings.copy()
cmdargs=parseArgs(sys.argv)
if not ignore_config in cmdargs:
    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(cmdargs)