Remove built-in vtk imports

[FezVrasta]

Right now this import is importing many heavy assets that are not always necessary.

https://github.com/Kitware/react-vtk-js/blob/master/src/index.js#L2

It'd be great if the library didn't import them and let the consumer import what they need.

[jourdain]

That would be a tricky one as one of the point of it is to allow direct usage. But maybe that list of import could be managed somewhere here assuming it gets loaded in the proper vtk.js namespace.

[FezVrasta]

Maybe you could expose an additional entry point with no auto imports?

import { View } from 'react-vtk-js/light';

[jourdain]

I like the light version. Let's go with that option then.