Constrain input for Step.options.buttons to an array of buttons.

[BrianSipple]

Closes #243

@rwwagner90 This should also fix our flaky buttons test that was occasionally failing on Travis.

[RobbieTheWagner]

This text is messed up. Got some random sentence fragments. The bullet points below also should not have been moved, as they are now in line with buttons, rather than nested under it.

[BrianSipple]

Eww... should be fixed now

[RobbieTheWagner]

Why remove this? We should keep the test functionality in place.

[BrianSipple]

I think it was wrong when I added it in the first place.

The hidden steps souldn't exist in the DOM at all. And also, Cy works in a way that it will try to wait until it finds something -- which causes it to hang here.

[RobbieTheWagner]

Unless we changed something, hidden steps definitely should exist in the DOM. Once a step is created and inserted into the DOM, it remains there, but is just set to hidden=true. In this case, it may not have been inserted into the DOM yet, but if that is the case, we should do a couple next/back clicks to make sure it is inserted into the DOM, and hidden when on another step. Not sure if we already have tests covering that or not.

[BrianSipple]

We set this.el to hidden, but then we also call hide on the tippy instance.

  hide() {
    this.trigger('before-hide');

    if (this.el) {
      this.el.hidden = true;
      // We need to manually set styles for < IE11 support
      this.el.style.display = 'none';
    }

    document.body.('data-shepherd-step');

    if (this.target) {
      this.target.classList.remove('shepherd-enabled', 'shepherd-target');
    }

    if (this.tooltip) {
      this.tooltip.hide();
    }

    this.trigger('hide');
  }

Since the former is contained in the later, it gets removed from the DOM