CLI interface refactor

- CLI_Command class is invoked from index.php to run Tasks

- Task factory takes a $name, and is executed with $params
  - CLI_Command::factory($task)->execute($params);

- Interfaces for standard I/O 
  - CLI_Input
    -  read()
    -  options()
  - CLI_Output
    - write()
    - writeln()
  - CLI_Options
    - Handles Task selection and params collection

- Backwards-compatible with 3.3 Task interface