Creates and initializes a Shell object.
function createShell(config, callback) {
  var shell = new Shell(config);

  modules.initialize(shell);
  extensions.initialize(shell);

  require('./commands').initialize(shell);
  require('./displayCommands').initialize(shell);
  require('./dataCommands').initialize(shell);

  process.nextTick(function() {
    callback(shell);
  });
}