Disabled buttons on Preview

Fixes [Issue #82](https://github.com/toopay/bootstrap-markdown/issues/82)
@toopay This is the code block for turning the preview off. The `this.enableButtons('all')` is causing the issue [#82](https://github.com/toopay/bootstrap-markdown/issues/82) that configured *disabled* buttons are being re-enabled. 

Adding the highlighted code will "re-disable" those initially disabled buttons (a quick fix)