Add the ability to specify a subfield

[yarkovaleksei]

I want to use this syntax:

@State('foo.nested.bar') bar

Now for these purposes I have to specify the function from which I return the field I need.

@State(state => state.foo.nested.bar) bar

[ktsn]

I'm not willing to add such feature because:

1. It's impossible to type check.
2. It would provide poor error message if there is typo on property name.
3. Vuex does not support that.