Add pop-up service

[ISO50]

Need some service that open an html pop-up at a given position. This could be a general component used at various positions throughout the diagram.

Usage Snippet

var options = {
  choices: {
    'a': { image: '...', className: '...', label: '....' },
    'b': { .... }
  },

  select: function(event, choice) {
    alert('user clicked choice');
  }
};

var popup = Popup.open(position, options);

// manually close
popup.close();

// automatically close on diagram click (?)

[ISO50]

Needed by bpmn-io/bpmn-js#180