Make invalid severity tolerance

```
Traceback (most recent call last):
  File "bin/vint", line 25, in <module>
    vint.main()
  File "/Users/####/Development/vint/vint/__init__.py", line 20, in main
    config_dict = _build_config_dict(env)
  File "/Users/####/Development/vint/vint/__init__.py", line 48, in _build_config_dict
    ConfigGlobalSource(env),
  File "/Users/####/Development/vint/vint/linting/config/config_file_source.py", line 12, in __init__
    self._config_dict = self.convert_config_dict(yaml.load(file_obj))
  File "/Users/####/Development/vint/vint/linting/config/config_file_source.py", line 22, in convert_config_dict
    cmdargs_dict['severity'] = Level[cmdargs_dict['severity'].upper()]
```