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

@param {String} app The app name as it's typed by the user (user/repo).
@param {Object} manifest Manifest containing application fields.
function (app, manifest) {
    if (manifest.type === undefined) {
      manifest.type = 'classic';
    }
    config.apps[app] = {
      name: manifest.name,
      displayName: manifest.displayName,
      version: manifest.version,
      description: manifest.description,
      type: manifest.type
    };
    configHelpers.saveConfig();
  }