Custom buttons based on 'a' tag not showing up

[Rudkovskiy]

Describe the bug
Custom buttons based on 'a' tag not showing up on tooltip

To Reproduce
Steps to reproduce the behavior:

1. Create button:

var a = document.createElement('a');
a.innerHTML = 'Button';
a.href = 'http://some-link';

3. Set button to custom buttons

 var tour = [ {
            element: '#some-element',
            title: 'Title',
            description: 'Description',
            buttons: [a],
        }];

Expected behavior
Link-button displayed in tooltip.

[Rudkovskiy]

PR: #147