Consider adding an option to control AccessMember's automatic object creation

`AccessMember` [automatically creates an object](https://github.com/aurelia/binding/blob/0.10.2/src/ast.js#L189) when assigning a value to a property on an object that doesn't exist.

[Some people love this behavior](https://github.com/aurelia/binding/issues/129). [Other people don't like it](https://github.com/aurelia/binding/issues/204).

Maybe we can make the behavior configurable?

cc @EisenbergEffect, @danfma

Start by reading the AccessMember implementation in src/ast.js around line 189, then review issues #129 and #204 for the arguments about automatic object creation. Define what configurable behavior should mean and how the option would be exposed; done should make assignment behavior selectable without changing the preferred default.

Written by the indexing model from the issue text.