Add option conflict feature

[jjulien]

Allow the user to specify one or more options that conflict with each other. For example, a script may have a verbose option and a quiet option. A user providing both of these options would be in conflict.

/**
 * @var CLIHelper\Helper $option
 */
// Working directly with option
$option->addOptionConflict($option)

// Working CLIHelper\OptionBuilder using the CLIHelper\Helper
$helper->newOption()
  ->withName("verbose")
  ->withLong("verbose")
  ->withShort("v")
  ->withConflict($option)
  ->build()

// Adding multiple conflicts using CLIHelper\Helper
$helper->newOption()
  ->withName("verbose")
  ->withLong("verbose")
  ->withShort("v")
  ->withConflicts(array($option1, $option2))
  ->build()

The user should only have to tell one option that it conflicts with another. The helper should take care of making sure both options know they have a conflict with the other.