Stop given plugin (unload it form cache and run its exit handleR).

@param pluginName The plugin name to start.
@param callback Termination.
function (pluginName, callback) {
    var config = configHelpers.getConfig();

    if (config.plugins[pluginName] === undefined) {
      logger.error('Plugin ' + pluginName + ' not installed!');

    } else if (pluginHelpers.loadedPlugins[pluginName] === undefined) {
      logger.error('Plugin ' + pluginName + ' not loaded!');

    } else {
      var options = config.plugins[pluginName];

      try {
        var plugin = pluginHelpers.loadedPlugins[pluginName];
        delete pluginHelpers.loadedPlugins[pluginName];
        nodeHelpers.clearRequireCache(configHelpers.modulePath(options.name));

        if (plugin.onExit !== undefined) {

          return plugin.onExit(options, config, function (err) {

            if (err) {
              logger.raw(err);
              logger.info('\t' + symbols.err + '\t' +
                          options.displayName + '');

            } else {
              logger.info('\t' + symbols.ok + '\t' +
                          options.displayName + '');
            }

            delete pluginHelpers.startedPlugins[pluginName];
            nodeHelpers.invoke(callback, err);
          });

        } else {
          delete pluginHelpers.startedPlugins[pluginName];
          logger.info('\t' + symbols.ok + '\t' +
                      options.displayName + '');
        }

      } catch(err) {
        logger.raw(err);
        logger.error('Plugin ' + options.displayName +
                     ' failed for termination.');
        return nodeHelpers.invoke(callback, err);
      }
    }
    return nodeHelpers.invoke(callback);
  }