add callback function support for generator scheduling

[dwarakaprasad]

depends on yeoman/yeoman-api#6
required by jhipster/generator-jhipster#25445

This PR allows a callback function as an option for ComposeOptions.schedule

The function receives the generator instance as the parameter and must return a boolean that will decide the scheduling. A true return value schedules the generator to be run under environment:run loop and a false queues the generator tasks immediately.

usage:

env.composeWith('hello:world', { schedule: (generator) => {
    // some code to decide the scheduling option
   return generator.immediate;
}});

[mshima]

Please add test for function returning false since a function is truthy.

current tests are in:

environment/test/environment.js

Lines 263 to 270 in 0218d49

	describe('passing false schedule parameter', () => {
	it('should not schedule generator', async function () {
	this.env.queueTask = sinon.spy();
	await this.env.composeWith('stub', { generatorArgs: [], schedule: false });
	if (isGreaterThan6(generatorVersion)) {
	assert(this.env.queueTask.calledOnce);
	assert(this.env.queueTask.getCall(0).firstArg !== 'environment:run');
	} else {