Feature: Add 'severity' Field to Output

[P-N-L]

Hello,

The CodeClimate specification marks 'severity' as an optional field,
but GitLab's JSON schema for artifacts:reports:codequality appears to mark it as required.

In order to improve the use of this tool with GitLab,
would it be possible to add the severity field to the output?
My thoughts are:

• It could be an option/flag to include it
• There could be either default severity value applied to all issues generated (maybe major?)
• It would be neat to offer an ability to map the flake8 error codes to specific severity values, but that could add significant complexity and maintenance burden

Thanks!