pre-* events #43
Description
It would be nice to have a chance before a model is actually inserted/removed from a collection or changed to enforce any collection invariants.
For example, say I have a collection of apple models, but I am only allowed to have one green apple at a time. On a pre-add I could check that the new apple is green and remove any existing green apple, and this would work regardless of what actually called the .add().
Likewise, if we had a pre-change event from ampersand-state, then the invariant could still be enforced when an apple changed from red to green, no matter where or what triggered the change.
Maybe a better example would be an invariant that there can be only a single model where selected is true.
(any pre-change event would be a matter for ampersand-state to implement, of course)