Add option to disable or change the resize property.

E.g.:
```js
$("#target-editor").markdown({
  resize: 'vertical',
})
```

It makes also possible to prevent appending this property in the markup by specifying an empty string (or null, false, undefined...).
This is very handy in case we want to style it directly from the CSS (resize direction, min/max-height, etc.) and avoid the usage of `!important`.

Ref. https://github.com/toopay/bootstrap-markdown/issues/26