Add --external-args option forwarded to script

[wallace11]

As we discussed earlier. It seems to work fine with just setting sys.argv. Tested with a script with argparse. I have no idea if it works with other libraries as well (click, etc...).

If there's something with the implementation that bothers you, please tell me and I'll make changes accordingly.

[wallace11]

Improved the implementation to work in the following manner:

1. load_script accepts an optional external_args argument.
2. If argument is provided, stores it as instance attribute.
3. Stores original sys.argv locally.
4. Fakes sys.argv by trying to write it with the external_args instance attribute.
5. Loads script.
6. Restores original sys.argv.

Also:

• I made sure the split was done using shlex.split() which allows proper character escaping.
• --external_args won't work now without arguments (no meaning to use it with empty args)