Add a new plugin to the configuration file.
The configuration file is written in JSON.
It adds an entry to the plugins field.
Name, display name, version and description are required.

@param {String} plugin The plugin name as it's typed by the user.
It s in the form user/repo.
@param {Object} manifest Manifest containing plugin fields.
function (plugin, manifest) {
    if (config.plugins === undefined) {
      config.plugins = {};
    }

    config.plugins[plugin] = {
      name: manifest.name,
      displayName: manifest.displayName,
      version: manifest.version,
      description: manifest.description
    };
    configHelpers.saveConfig();
  }