overriding open/close actions

[cantsin]

For this library to be more useful, we need a better way to control which nodes are visible from outside. Currently we use a list of openNodes and an on-open action to achieve this, but this is not ideal:

• Relies on an undocumented action to control the list of open nodes.
• If we want to open all children at once by using on-open, we implicitly assume children are already loaded.
• Does not deal with the collapsing case unless we add a corresponding on-close action.
• Has some API leakage. (see API leakage with the on-open action #8)
• Need to accommodate a variety of use cases: open all children recursively; allow collapsing iff certain criteria are met.

Right now, actions are global in the sense that they are set per async-tree invocation. Should we make them per-node, perhaps? This might alleviate all of the problems above (sans documentation).