Feature: Add strict state usage

[grandevel]

Currently it is possible to 'pollute' state by setting any property without it being first defined - state.blah = ''xxx', either in the root namespace or sub namespaces. Add a config parameter such that properties cannot be added to state when in 'strict mode', unless it is a sub property of an existing state item.

Examle:

const state = {
foo: myObject,
bar: 6
};

state.cheese = 'xxx' would fail/cause an exception
state.foo.cheese = 'xxx' would succeed because it is in a subobject