Recognize error codes from Flake8 extensions

Summary:
Flake8 extensions are allowed to use their own letter-prefixed codes. For
example, the flake8-debugger extension emits 'T002'-tagged messages.

This change relaxes getLintCodeFromLinterConfigurationKey() to also recognize
extension codes. Otherwise, attempting to configure message severities for
e.g. 'T002' would result in an exception.

Messages from extensions continue to default to ERROR severity, as they did
before this change.

Test Plan:
Successfully reduced the severity of 'T002' to a warning via .arclint:

"severity": {
    "T002": "warning"
}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15810