Added a cli infrastructure

CliTool is a base class for command-line interface tools (such as the plugin manager and potentially others). It supports the following:
  - single or multi command tool
  - help printing infrastructure (based on help files)
  - consistent mechanism of parsing arguments (based on commons-cli lib)
  - separation of argument parsing and command execution (for easier unit testing)
  - terminal abstraction (will use System.console() when available)
could potentially be replaced by an inlined guava `Function<Void, T>` in `Streams`, so this class is not needed?