Add support for data attributes on buttons

[zeckdude]

Regarding Issue #307, I have implemented the changes and it works great.

To reiterate, this is how it is implemented:

var sessionModal = new BootstrapDialog({
  title: 'Please provide details about the class',
  message: 'Hi there',
  buttons: [
  {
    id: 'delete-session-prompt-btn',
    label: 'Delete Class',
    cssClass: 'btn-danger pull-left hidden',
    data: {
      'js': 'delete-btn',
      'ui': 'something'
    }
  },
});

This results in data-js="delete-btn" and data-ui="something" on the button element.

[nakupanda]

Merged, thanks for the implementation!

[nakupanda]

Please also create an example in the example page, if possible, thanks :)