Support custom renderers
Hello 👋 

**Motivation:** In my current company we want to try `react-sketchapp`. Although, it seems that in it's current shape, react-sketchapp supports only a limited subset of React Native components. I'm not sure if its a problem for us, but I think that having an opportunity to create custom renderers will help us to overcome potential issues in the future.

**Issue:** In its current state, `react-sketchapp` can't be extended with custom renderers.

**Proposed solution:** Create a mechanism that injects custom renderers. As I see from the code, the only place you use renderers is in `flexToSketchJSON`. This function is used in file `render.js`, function `render`. So my proposal is to supply a third argument to the `render` function: 
```js
(element: React$Element<any>, container: SketchLayer, renderers: Renderers): ?SketchLayer
```
Thus, `flexToSketchJSON` won't import `./renderers` from itself, but use a given parameter instead. 

Later, in user project, in the file manifest is pointing to, supply a third argument to the `render` like this:

```js
import customRenderers from './customRenderers';
//...
render(<Document colors={colorList} />, context.document.currentPage(), customRenderers);
```

Of course, in order not to break things, third argument should be optional. This is how I see `render.js`:

```js
import renderers from './renderers';
//...
export const render = (element: React$Element<any>, container: SketchLayer, customRenderers: Renderers): ?SketchLayer => {
  if (appVersionSupported()) {
    try {
      const tree = buildTree(element);
      return renderToSketch(tree, container, { ...rendereres, ...customRenderers });
    } catch (err) {
      const tree = buildTree(<RedBox error={err} />);
      return renderToSketch(tree, container, renderers);
    }
  }
  return null;
};
```

Please, let me know if you like the proposed approach so I can compose a PR