Control layout and styling in Build Annotations

On July 5th, 2018, Buildkite introduced significant improvements to their annotations feature. These enhancements include a CommonMark compliant parser for markdown rendering and support for GitHub Flavored Markdown features such as shorthand table syntax and strikethroughs. Additionally, the styling of many HTML elements has been improved, making them appear more organized. A range of CSS classes based on Basscss have also been added to provide layout, type, and other styling controls within annotation bodies. Lastly, a small Annotation Tester pipeline was created to help users experiment with annotation formatting for both Markdown and HTML markup. Users can refer to the buildkite-agent annotate documentation to get started with these new features.