Add alias schema option

[rkt2spc]

Summary
Supporting alias is a great feature for an ORM. Currently, we already have virtual getter/setter, I just incorporate them into a schema option for usability

var schema = new Schema({
  foo: { type: String, alias: 'bar' } // Create virtual getter/setter
});

Test plan

var Schema = mongoose.Schema;
var TestSchema = new Schema({
  foo: { type: String, alias: 'Faa' },
  bar: {
    baz: { type: String, alias: 'Baa' },
    kar: { type: String, alias: 'Kaa' }
  }
});
var Test = mongoose.model('Test', TestSchema);
var test = new Test({
  foo: 'a',
  bar: {
    baz: 'b',
    kar: 'c',
  }
});

assert.equal(test.foo, test.Faa);
assert.equal(test.bar.baz, test.Baa);
test.Kaa = 'd';
assert.equal(test.bar.kar, test.Kaa);

[rkt2spc]

Plan to incorporate alias into query and constructor

[vkarpov15]

Thanks, this is a very neat solution to a feature we've wanted for a while. Will merge with 4.10.0 (ETA April 30)

[rkt2spc]

Lol, checks fail on "model indexes auto creation can be disabled". Hmm...

[rkt2spc]

Close and reopen the PR to trigger a rebuild now reporting no error