Fix config.toml property for logging always being overwritten by argparse default

[ahester57]

Enable default WARN log level while adding support for property-less config file.

• Remove default from argparse --warn.
• Add NoneGuard to logger.start().
  • Enables default behaviour when given level=None.
• Utilize dict.get() when accessing config.toml's "logging" property.
  • Increases flexibility of property file.
  • No longer required to have logging as a property.