Async loading of dash-vtk for multi-page apps

[xhluca]

Since the bundle for dash-vtk is quite large (1MB), it might be beneficial for multi-page apps (such as dash-docs) if dash-vtk is loaded asynchronously, such that people that do not visit the page(s) featuring dash-vtk don't have to load the bundle.

I think this PR shows how to add async components: plotly/dash-core-components#739

^^ @chriddyp correct me if it's not the correct PR

[xhluca]

If this doesn't involve changing the source code for dash-vtk I should be able to handle it.

[xhluca]

Ok so it seems it only works when there's a default export: https://reactjs.org/docs/code-splitting.html#named-exports

Might need to change some imports in this case.