[zfleeman]

Closes #114.

A config file used to win whenever a command-line value matched argparse's built-in default, so --config my.json -c x264 --no-approx still encoded with x265 and kept --approx on.

Now _merge_config_args() loads the config file(s), passes their values to parser.set_defaults(), and parses the command line again. The order is command line > config file > built-in defaults, and anything you type wins.

Notes:

• update_args_from_config() and its == parser.get_default(k) check are removed.
• This reuses the first full parse to find the config file, instead of a separate parse_known_args() step. _search_for_default_config() doesn't change.
• Config keys that aren't flags still end up in the args, and from/to still go through _extract_times().
• A "config" key inside a config file is still followed. The outer file wins over the one it points to.

Tests in test_arguments.py cover -c x264, --no-approx, -s 20, and -a 64 beating a config file, plus config values applying when the flag isn't passed and unrelated flags keeping their built-in defaults.