Add tag hooks for mount / unmount

[somebee]

We need something like willMount, didMount, willUnmount (see lifecycle methods in react). Hooks should be called when the node is actually mounted in the document (not when it is appended to a parent).

There are a few different ways to implement it. Performance is a primary concern. Since almost all tags will not have such hooks I think we should look for them in defineTag/extendTag and then add some custom logic / override certain methods if present. More specific implementation ideas will be posted here.