feat(getMarkupFromTree): Add onBeforeRender handler.

[avocadowastaken]

Usage of getMarkupFromTree create duplicates in some React libs like react-head.

const headTags = [];

const app = await getMarkupFromTree({
  onBeforeRender: () => {
    // Clear tags before each render.
    headTags.splice(0, headTags.length);
  },
  renderFunction: renderToString,
  tree: (
    <HeadProvider headTags={headTags}>
      <App />
    </HeadProvider>
  )
});

[trojanowski]

Thanks @umidbekkarimov