Add 'connected' event for custom elements

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

Adds an event connected that is emitted once a custom element has connected.

myCustomElement.addEventListener('connected', function() {
    // do things once the element has connected
});

Resolves #754