Add pendReady() to factor out repetitive code and facilitate use of the mechanism in plugins

[jmm]

This is a proposal to factor out this pattern that's used in several places into a method:

self._pending ++;
somethingAsync(function callback () {
  // ...
  if (-- self._pending === 0) self.emit('_ready');
});

So this instead:

somethingAsync(this.pendReady(function callback () {
  // ...
}));

The reason I didn't name the method _pendReady is because I want to suggest exposing / advertising it to facilitate creating plugins that need to do some kind of async setup. For example, create a plugin that needs to register a transform in a specific position, but needs to do some async setup before the transform can be invoked, e.g.:

function plugin (b, opts) {
  var real_transform;

  function placeholder (file) {
    return real_transform(file);
  }

  b.transform(placeholder);

  something_async(b.pendReady(function () {
    real_transform = ...
  }));
}

If you approve I can write some documentation.