Function option for start, serial, and parallel

[timhall]

Allowing function as argument to start, serial, and parallel would allow tasks to be composed without having to export everything including tasks that are not meant to be called directly.

function * build(task) {
  yield task.parallel([js, css]);
}

// "internal" functions 
function * js(task) {}
function * css(task) {}

module.exports = {
  build
};

[lukeed]

Thanks! :raised_hands: