Add option to hide passed tests

What does this MR do and why?

Add option to hide passed tests in the table posted to Slack.

Closes #75 (closed)

Screenshots or screen recordings

Where /Users/ddavison/Downloads/rspec-*.xml returns:

• rspec-sample-1.xml
• rspec-sample-2.xml
• rspec-sample-3.xml
• rspec-sample-4.xml
• rspec-sample-5.xml

Before	After

How to set up and validate locally

CI_PROJECT_ID=278964 \
CI_PIPELINE_ID=1332326871 \
bundle exec post-to-slack \
  --slack-webhook-url "$CI_SLACKHOOK_URL" \
  --channel "qa-" \
  --username "GitLab QA Bot" \
  --icon-emoji "ci_running" \
  --message "TEST" \
  --sort-by "Failures" \
  --sort-direction "desc" \
  --include-summary-table "/Users/ddavison/Downloads/rspec-*.xml" \
  --gitlab-api-token "$GITLAB_API_TOKEN" \
  --include-failed-jobs-table \
  --hide-passed-tests

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.