Release v0.18.0

[andymeneely]

Be sure to remember to do the following for releases.

• [x] CHANGELOG is written for all new changes
• [x] README is updated
• [x] Samples are updated
• [x] Are all the dependencies up to date?
• [x] Check sample_regression_spec.rb regression tests are all enabled (i.e. overwrite_sample is commented out)
• [x] Bump version.rb
• [x] Bump docs/conf.py versions
• [x] Do a full rake locally
• [x] rake sanity, and check visually
• [x] GitHub Actions is passing on dev branch
• [x] Merge master branch
• [x] Merge release branch
• [x] Push master and release branches
• [x] Create GitHub release tag
• [x] gem push pkg/squib-x.y.z.gem
• [x] Github milestone closed
• [x] Activate version on squib.readthedocs.org (Versions)
• [x] Set default version on squib.readthedocs.org (Advanced Settings)
• [x] Bump version.rb to the next alpha
• [x] Publish on BoardGameGeek thread
• [x] Check Docker build on Dockerhub: https://hub.docker.com/repository/docker/andymeneely/squib
• [x] Check Docker docker pull: docker pull andymeneely/squib:latest
• [x] Check Docker build locally in samples: docker run --rm -v "%cd%":/usr/src/app andymeneely/squib:latest ruby basic
• [x] Anything else to add to this checklist?