Enable to collect inital class properties as vue instance data

[ktsn]

This PR enable to declare initial data as class properties for components. This let us define Vue component more native class-like style.

Example:

@Component({
  props: ['foo']
})
class MyComp extends Vue {
  bar = 123
  baz = this.foo
}

...will be converted into:

{
  props: ['foo'],
  data () {
    return {
      bar: 123,
      baz: this.foo
    }
  }
}

[HerringtonDarkholme]

I found this approach is problematic when users define new properties in custom beforeCreate lifecycle hooks. Here it only collects props and custom properties get lost.

[ktsn]

Thanks for your review! I'm thinking this problem but maybe we can solve this by using vm as the prototype for data instead of Component.prototype?

[HerringtonDarkholme]

Rather than whitelisting props above, it might be better to blacklist property keys which should not be collected.

[ktsn]

Fixed the dummy instance's prototype to actual vm. The dummy should have props and other properties defined in beforeCreate.

[yyx990803]

That's smart. I wanted to do this in the original version!

[michaelkrupp]

@ktsn any idea on when this will be updated on NPM?

[ktsn]

@michaelkrupp Maybe after #30 is merged?

[ktsn]

I noticed this causes error if it is used with Babel because the dummy object does not inherit component class constructor.
I'll fix it ASAP.