added postInitialize

[bhamblok]

Now you can use a "postInitialize"-function which will be triggered just AFTER all "initialize"-functions from parent -states and -collections have been triggerd.
This is necessary when you need to access some (non-binding) properties from a parent-State-object which needs to be altered first in it's own initialize function.

[bhamblok]

Hi @latentflip and @pgilad. I created a new branch to be able to replace the previous commits. I also updated the readme-file so it is more clear why I would like to have this functionality added to ampersand. Feel free to ask more information.

Thanks

[pgilad]

Hey - see my comments for AmpersandJS/ampersand-state#176 (comment) regarding this topic.

[jrmyio]

Kind of in need for a function like this.

Let's say I want to do a calculation on a collection as soon as the collection is initialized or changes. Currently in initialize() the collection is not ready yet.