Add support for subcommands in CLI

[oleg-nenashev]

Jenkinsfile Runner includes more options and behaviors, and it is difficult to maintain it with a single-level CLI interface. I suggest introducing sub-commands so that we can easily evolve CLI and isolate commands so that it is easier to use and maintain JFR. Examples of commands:

• jenkinsfile-runner [run] [args...] for common mode
• jenkinsfile-runner cli [args...] for CLI
• jenkinsfile-runner manage-plugins [args...] for the plugin manager
• jenkinsfile-runner help for the top-level help
• jenkinsfile-runner version for the version info

Acceptance criteria:

• There is a two-level CLI implemented. It might require switching from args4j to another lib
• It is possible to add new commands to the bootstrap
• Nice2have: It is possible to add more sub-commands from classpath (e.g. via declaring a system property or an annotation)

P.S: This is a spin-off of the conversation with @literalplus in #213 : #213 (comment)