async process () {
    if (this.context.cwd === this.outDir) {
      throw new Error('Unexpected option: "outDir" cannot be set to the current working directory')
    }

    this.context.resolveCacheLoaderOptions()
    await fs.emptyDir(this.outDir)
    logger.debug('Dist directory: ' + chalk.gray(this.outDir))
    this.prepareWebpackConfig()
  }
Doing somthing before render pages, e.g. validate and empty output directory,
prepare webpack config.

@returns {Promise<void>}
@api public