I was hoping to make a mincer plugin for [Rollup](https://rollupjs.org/), however their API is promise based. Could you support this in `Template.prototype.evaluate`? Right now, if I specify a Promise based function for `evaluate`, when Mincer invokes `evaluate`, it won't wait for the Promise to resolve. Also see this Stackoverflow question: http://stackoverflow.com/questions/21424867/run-an-asynchronous-stream-based-function-synchronously-in-node-js