Added promise support for external API

[orangemug]

No description provided.

[orangemug]

@oliverbrooks maybe we should we remove support for callbacks all together... thoughts?

[oliverbrooks]

Looking good. Got a comment which is a matter of preference on how to use promises.

I like to have a coroutine which is uses purely promises or purely callbacks. Responding to both is then handled at the end of the function.

For example, with promises:

function readFile (filepath) {
  return new Promise(function (resolve, reject) {
    fs.readFile(filepath, function (err, raw) {
      if (err) {
        reject(err);
      } else {
        resolve(raw);
      }
    });
  });
}

module.exports = function (file, opts, done) {
  return readFile(file)
  .then(function (file) {
    // Do something promisey
  })
  .then(function (file) {
    // More promisey stuff
  })
  .then(function (result) {
    // Handle success for callback and promise
    if (done) {
      done(null, result);
    } else {
      return result;
    }
  })
  .catch(function (err) {
    if (done) {
      done(err)
    } else {
      throw err;
    }
  })}
}

[oliverbrooks]

Regarding removing support for callbacks. I haven't got anything against that anymore.

It's essentially that either you include the promise > callback boilerplate for everyone or those who need it add it to their code. Pretty much everyone is familiar with how to convert promise to callback nowadays.