Support sync rendering even of async transforms

[ForbesLindesay]

We could use sync-rpc to render transforms that only support async, synchronously. It would be relatively slow, but if you need it then at least it would work.

The code would look something like:

worker.js

const jstransformer = require('./');

function init(transformName) {
  const t = jstransformer(require('jstransformer-' + transformName));
  return function render({str, options, locals}) {
    // sync-rpc converts this promise into a synchronous result
    return t.renderAsync(str, options, locals);
  }
}

module.exports = init;

index.js

const syncRPC = require('sync-rpc');

// ...

Transformer.prototype.render = function (str, options, locals) {
  if (!this.can('render') && this.can('renderAsync')) {
    if (!this._syncWorker) {
      this._syncWorker = syncRPC(require.resolve('./worker.js'), this.name);
    }
    return this._syncWorker({str, options, locals});
  }

  // ... normal sync implementation here ...
}

// ...