Add an option `uiDirs`

The `uiDirs` option allows users to provide their own set of directories
with UI files, e.g. to provide a custom swagger-ui fork and a custom
set of style/font overrides:

    explorer(app, {
      uiDirs: [
        path.resolve(__dirname, 'public'),
        path.resolve(__dirname, 'node_modules', 'swagger-ui')
      ]
    });

The existing option `swaggerDistRoot` is deprecated now.