Function promisify

lib/utils/plugin-adapter.js:55–70

const bind = (fn, plugin) => fn.bind(plugin);
const promisify = returnsFile => fn => {
  switch (fn.length) {
    case 1:
      // Modern API:
      return fn;
    case 2:
      // Legacy API:
      return returnsFile ?
        microPromisify(fn) : // fn(file, callback) => void
        file => fn(file.data, file.path); // fn(data, path) => Promise
    case 3:
      // Legacy API: fn(data, path, callback) => void
      const promisified = microPromisify(fn);
      return file => promisified(file.data, file.path);
  }
};

const thenify = fn => {
  return file => Promise.resolve(file).then(fn);