Provide mapping for severities and default severity

[sk-]

Some linters like pep8 or rubocop print the severity with a code or with just a letter.
For example pep8 has E123 and W456, whereas rubocop has C, W, E among others.

One alternative to normalize this would be to have something like this in the configuration file:

severities:
  - error: "E\\d+"
  - warning: "W\\d+"

Additionally, some linters do not provide a severity. In such cases it would be good to provide a default one.

default_severity: error