Add Severity Rating Feature Based on CVSS Score

[dfritsch86]

The standard officially defines severity ranges of CVSS Vectors based on their score, i.e. low, medium, high, and critical. See here for reference: https://www.first.org/cvss/specification-document#5-Qualitative-Severity-Rating-Scale

It would make sense to add this to the gem directly, as it is part of the standard.

Mir freundliche Grüse!