wip: add vulners plugin

[antongolub]

Resolves #43

Changes

{
  plugin: [['npm-registry-firewall/vulners', [{
      relevance: 95,  // Optional. Lower bound on the search result (lucene score 0...100) for associating the found CVEs with the target package. Defautls to 90
      cond: 'or',     // Optional. Defaults to `and`
      policy: 'warn',
      'cvss.score': 60,
      'cvss3.cvssV3.availabilityImpact': 'MEDIUM'
    }, {
      policy: 'deny',
      'cvss.score': 95,
      'cvss3.cvssV3.availabilityImpact': 'CRITICAL'
    }],
  ]]
}

• New code is covered by tests
• All the changes are mentioned in docs (readme.md)