Public release + DSL improvements

This is the first public release of the plugin! Check out the plugin at the Gradle Plugins portal: https://plugins.gradle.org/plugin/com.widen.buildkite

There are a few breaking changes here as well as some quality-of-life improvements.

Breaking changes

• The gradleStep and publishDocsStep DSLs have been removed, as they assumed Widen-specific practices and patterns. Widen projects are recommended to replace these with the appropriate Buildkite plugins instead.
• The commandStep.plugin DSL no longer accepts two string arguments for a plugin version because it was confusing and unclear when to use the single argument vs. the double argument. Plugins are now always specified with a single string that follows the Buildkite naming convention for referencing a plugin (version is specified after a hash #).
• buildkite.agentQueue has been renamed to buildkite.defaultAgentQueue.
• The plugin ID changed from widen.buildkite to com.widen.buildkite.

Added

• Standalone pipeline scripts (.buildkite/pipeline*.gradle) now define pipelines automatically based on the camel-case version of the filename, and the script evaluates inside the context of the pipeline automatically. Generally this means you can just remove any top-level buildkite.pipeline() calls.
• The commandStep.plugin DSL now also accepts closures as a way of configuring a plugin in addition to maps. This results in a much more natural looking configuration of plugins.