Investigate support for separate WebAssembly instantiation

[mdaines]

Currently Viz.js bundles its WebAssembly code, which it passes to the Emscripten module for instantiation with WebAssembly.instantiate. This means that a user of Viz.js can't call WebAssembly.instantiate separately to instantiate the WebAssembly module which would reduce the total download size and be useful for running the library in a worker.

In order to support this, it looks like Viz.js would need to make the importObject used by the Emscripten module accessible somehow.