message: fix(preset-umi): use rootElement from contextOpts
diff: @@ -37,7 +37,7 @@ async function render() {
  routes,
  routeComponents,
  pluginManager,
  - rootElement: document.getElementById('{{{ mountElementId }}}'),
  + rootElement: contextOpts.rootElement || document.getElementById('{{{ mountElementId }}}'),
  {{#loadingComponent}}
  loadingComponent: Loading,
  {{/loadingComponent}}
  