Add `setMeta()` method to Node

[danilopolani]

The PR implements a setMeta() method for Nodes to chain it in the component builder:

class MyComponent extends Rete.Component {
    constructor () {
        super('Component name');
    }

    builder (node) {
        return node
            .setMeta({ foo: 'bar' })
            .addControl(...)
            .addInput(...)
            .addOutput(...);
    }
}

[ChBernat]

Any idea when will it be merged?