Function option for start, serial, and parallel

Nobody has claimed this yet.

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.

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

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

module.exports = {
build
};
```

Start by locating the implementations and existing tests for start, serial, and parallel. Check how exported tasks are currently invoked, then define the behavior for passing generator functions directly; done means the shown build example composes internal js and css functions without exporting them and the relevant tests pass.

Written by the indexing model from the issue text.