add a Watcher generator

[palazzem]

Wheelie could offer a public API to generate a watcher for a Task. Suppose we have a task that should be triggered everytime a file changes in a folder, it's something that only the Task itself could know.

I propose to have a config attribute that if is found during the Wheelie build, a new watcher is activated. For instance, a SASS task could:

function config(globals) {
  var source = globals.src + 'scss/**/*.{sass,scss}';

  return {
    watcher: source,
    src: source,
    dest: globals.dest + 'css/',
    sourceComments: 'map',
    imagePath: 'images/',
    autoprefixer: ['last 2 version']
  };
}

At this point the Wheelie#add() method should add the task to the watchers list in the _registry, and a real watcher task must be generated using this list.
If we achieve this architecture, we can also set the task watch as a reasonable default for Wheelie.