Build the webpack configuration
@param  {String} wantedEnv The wanted environment
@return {Object} Webpack config
function buildConfig(wantedEnv) {
  let isValid = wantedEnv && wantedEnv.length > 0 && allowedEnvs.indexOf(wantedEnv) !== -1;
  let validEnv = isValid ? wantedEnv : 'dev';
  let config = require(path.join(__dirname, 'cfg/' + validEnv));
  return config;
}