Provide a Maven plugin for generating help during a build

[rvesse]

It would be useful in some scenarios to have help generated from an Airline powered CLI during the Maven build process. Ideally this would be done as a Maven plugin so that we could run automatically during the build without having to resort to some indirection (e.g. using the Maven Ant-run plugin)

The plugin should have the following characteristics:

• [x] MUST be run in the package phase (or later) since it will need access to the compiled code in order to generate help
• [x] MUST support specifying which classes to generate help for
• [x] MUST support generating global, group and command help
  • [x] If group/command help is requested for a CLI then pass in each group/command from the CLI to the help generator
  • [x] SHOULD allow for configuring this as desired e.g. generate help for a CLI uses a command help generator
• [x] MUST allow for specifying which help generator to use
  • [x] SHOULD have sensible default behaviour
  • [x] SHOULD provide a simple <style> setting that specifies one of the built-in help generators i.e. cli, man, html, markdown
  • [x] SHOULD allow for pulling in custom help generators that are not otherwise dependencies of the code being built
  • [x] MUST allow for passing in parameters to the help generator e.g. whether to include hidden groups, command, options and arguments
• [x] MUST allow for specifying the output directory
• [x] MAY allow for generating multiple kinds of help at the same time

[rvesse]

Working plugin included with 2.5.0 release, leaving this open as there are a couple of items still to implement

[rvesse]

Completed as part of 2.7.0 work