CLI option to specify config file location

Code to add an option which allows to specify a location for a config file to be used with `-c` or `--config`. Both `PixivConfig.loadConfig()` and `PixivConfig.writeConfig()` respect that choice.

I personally use PixivUtil2 with a bash script that periodically downloads new art from my followed artists and currently PixivUtil2 creates a new config.ini file every time the script runs. This should remedy that and allows to use different configurations if needed (in scripting, for example).