Add support for data attributes for custom buttons (dialog)

POC for #156
Something like this - to be able to set additional data attributes for custom dialog buttons. This way we can localize 'em using L20n (for example)

```coffeescript
    showChangelog: (text) ->
      html = "<pre class='changelog'>#{text.replace(/\n/g, '<br/>')}</pre>"
      dlg = bootbox.dialog(html, [{
          'label' : '__ACK__',
          'class' : 'btn-success',
          'data': { 'l10n-id': 'ack_changelog_action' },
          'callback': => @acknowledgeVersion()
        }, {
          'label' : '__LATER__',
          'data': { 'l10n-id': 'later_changelog_action' },
        }],
        { className: 'changelog' })
      document.l10n.localizeNode($('.bootbox').get(0))
```

P.S.
I'm still using Bootbox 3.x so - PR is for v3.x branch.