Async plugins

[Paul Miller]

Dupe of #2 but I got an actual API proposition

What about this

// sync
rework(css).use(plugin).toString()

// async; promise-based
rework(css)
  .then(function(object) {
    return plugin(object);
  })
  .then(function(object) {
    return object.toString();
  })

// or similar

@necolas

[Paul Miller]

tired of dreadful performance of synchronous FS calls — this would really help in case of @imports etc

[Rob Loach]

Just sticking your PR in here so people can follow along: #185