Option to adjust height of the editor

First requested by [androgeninc](https://www.reddit.com/user/androgeninc/).

Currently you can only adjust the height by adding css.
```
#wmd-input {
    height: 100px;
}
```
Request is for an option to adjust the height of the editor from Jinja2 itself. [Flask-Pagedown](https://github.com/miguelgrinberg/Flask-PageDown) has an option for this:
```
{{ form.pagedown(rows=10) }}
```
