[Feature] Configuration file

[moonburnt]

It could be nice to have some configuration file (say, in ini format) that can be used to override some default variables automatically (e.g instead of using command-line options each time). Example options that may be tweaked this way:

• Directory used to store games.
• Default OS, for which game will be downloaded (win/mac/lin)
• Should game's goodies be downloaded or not

The logic is following:

• If config.ini doesnt exist, script makes one with default values
• If config.ini exists, but not every necessary variable present there - for missing variables, script falls back to defaults
• If config.ini contains incorrect variables - script throws error and sys.exit(1)
• If config.ini exists and is correct - variables stored inside are used instead of script's defaults
• If script has been launched with additional flags that affect the same things as config.ini - options described in flags will be used instead of options in config.ini