Add sync API

[aduh95]

Adds a sync version of the renderString method:

const vizRenderStringSync = require("@aduh95/viz.js/sync");
console.log(vizRenderStringSync("digraph{1 -> 2 }"));

Key differences with async API:

• It uses asm.js instead of WebAssembly, this should come with a performance
  hit and a bigger bundled file size.
• It is a CommonJS module, while the rest of the API is written as standard
  ECMAScript modules. The upside is this syntax is supported on a wider Node.js
  version array.

Fixes #1