Custom nodes - plugins

[Idered]

All current nodes should be in form of a plugin.

Example: nodes.sequence() returns an object without function to interpret it. If we want pluggable nodes then it should contain interpretation logic.

Here are some ideas after pluggable nodes will be implemented:

1. Move default nodes to separate package @btree/nodes
2. @btree/core should only contain functions for creating nodes and executing tree
3. Allow people to create custom set of nodes
4. Default nodes implementation should be optional
5. If default nodes will be in separate package then nodes.root should not be a node but a normal function eg. createTree - it should be in core package.

Implementing this is crucial for library future. People have different needs and there's no way to fulfill them all. With solid plugin system we could keep this lib alive.