Numerous updates to the Spring CLI, primarily for better embedded REPL
shell support:

* Refactor the CLI application to help separate concerts between the
  main CLI and the embedded shell. Both the CLI and embedded shell now
  delegate to a new `CommandRunner` to handle running commands. The
  runner can be configured differently depending depending on need.
  For example, the embedded shell adds the 'prompt' and 'clear'
  commands.

* Most `Command` implementations have been moved to sub-packages so that
  they can be co-located with the classes that they use.

* Option commands are now only used in the CLI, the embedded shell
  does not user them and details have been removed from the Command
  interface.

* The REPL shell has been significantly refactored to:
    - Support CTRL-C to cancel the running process. This is supported
      when running external commands and most internal commands.
    - Fork a new JVM when running commands (primarily for CTRL-C support
      but also for potential memory and classpath issues)
    - Change the "continue" trigger from `<<` to `\`
    - Support command completion of files
    - Add ANSI color output
    - Provide 'help' support for internal commands (such as 'clear')
    - Remove the now redundant `stop` command

Fixes gh-227