Implement Atom save hook

[bantic]

Save hook is on the atom's env and accepts arguments: (value, payload).
Calling the save hook rerenders the atom.

Example:

let atom = {
  name: 'my-atom',
  type: 'dom',
  render({env, value, payload}) {
    let el = document.createElement('button');
    let clicks = payload.clicks || 0;
    el.appendChild(document.createTextNode('Clicks: ' + clicks));
    el.onclick = () => {
      payload.clicks = payload.clicks || 0;
      payload.clicks++;
      env.save(value, payload);
    };
    return el;
  }
};

Also: improve postAbstract buildFromText to accept data for an atom,
i.e. "abc@(...jsondata...)", e.g.:

buildFromText('abc@("name": "my-atom", "value": "bob", "payload": {"foo": "bar"})def');
// -> "abc" + atom with name "my-atom", value "bob", payload {foo: 'bar'} + "def"

Fixes #399

[bantic]

released in v0.10.8