Allow for lists as values in excludes mappings

[raydouglass]

Updates the excludes parsing so that values of the mappings can be either Strings or Lists. This allows for more concise and readable complex excludes. The original behavior (Map<String, String>) continues to work.

I updated the README.md and unit tests for the changes.

Examples (further details in readme changes):

# axis2.yml
exclude:
  - RUBY_VERSION: 2.1.8
  - RUBY_VERSION: 2.3.0
    DATABASE: 
      - oracle
      - mysql

# axis3.yml - cartesian product of both lists
exclude:
  - RUBY_VERSION: 
      - 2.1.8
      - 2.3.0
    DATABASE: 
      - oracle
      - mysql

[mike-wendt]

Super excited to see this as it simplifies our exclusion lists immensely. Thanks @raydouglass

[sue445]

I released as 0.3.0 🚀
https://github.com/jenkinsci/yaml-axis-plugin/blob/master/CHANGELOG.md#yaml-axis-030-2020-11-21