Minor bugfixes, ignore_packages option

1. User can now ignore packages what either has no license or he/she is sure these packages are not a threat.

 "ignored_packages": {
    "python_pipenv": [
      "PyInquirer",
      "setuptools"
    ],
    "npm": [],
    "maven": [],
    "yarn": []
  }

2. User can now run this program as python3 -m license_sh