Support load custom config file (name, location)

[nhymxu]

Currently, script read config.ini from current directory.
But sometime I want load config file with other name, or other location.

Example:

python cf-dns-update.py other-config-file.ini
python cf-dns-update.py /etc/cf-dns-update/config.ini

Using built-in library argparse, script can run smooth without install package