Introduce lifecycle hooks for models about mobx-state-tree

[RainerAtSpirit]

Naming convention before/after resonates better with me.

[mattiamanzati]

+1

afterCreateInstance to setup reactions! :D

[mweststrate]

Proposal:

• preprocessSnapshot(snapshot) => snapshot (called before applying a snapshot, both during create and later on)
• postprocessSnapshot(snapshot) => snapshot (called when creating a snapshot)
• didCreate() called after instantation, but not during reconcilation
• didAttach() called when node gets attached to a tree (not called if attached before creation)
• willDetach() called before detaching from a tree
• willDestroy() called before a node is killed

[mattiamanzati]

@mweststrate Maybe add a preprocess and postprocess for patches too?

[mweststrate]

Merged by #98 and released as 0.3.3.

Following lifecycle hooks are now available:

• afterCreate
• beforeDestroy
• afterAttach
• beforeDetach

More lifecycle hooks should be demand / use case driven. E.g. processing snapshots.

Also introduced the convenience method addDisposer which is useful in afterCreate, avoid the need to setup a beforeDestroy just to fire disposers.

E.g.:

const Todo = types.model({
   afterCreate() {
       addDisposer(this, reaction(() => getSnapshot(this), s => submitSnapshotToServer(s)))
   }
})