pep8 parser to log high severity fatal errors

Looking at the source of pep8 parser, seems to be only able to set med or low in severity.

```
/**
* Returns the Severity level as an int from the PEP 8 message type.
*
* The different message types are W for warning and E for error.
* Because these are style guide warnings we set the severity
* values lower than what the apparent values would be.
*
* @param messageType the type of PEP 8 message
* @return an int is matched to the message type.
*/
private void setServerityLevel(Violation violation, String messageType) {

switch (messageType.charAt(0)) {
case 'E':
violation.setSeverity(Severity.MEDIUM);
violation.setSeverityLevel(Severity.MEDIUM_VALUE);
break;
case 'W':
default:
violation.setSeverity(Severity.LOW);
violation.setSeverityLevel(Severity.LOW_VALUE);
break;
}
}
```

Would it be possible to add HIGH level for FATAL errors like:

```
foo/bar/utils.py:166:11: F821 undefined name 'foo'
```

So for ones that start with F, make them high.