Allow template to be optional

My use-case is to generate HTML partials for an Angular app.  I'm lazy, so I'd rather avoid creating a simple template like:

```html
<%= content %>
```

Instead, I'd prefer to be able to specify the template as `false` and be done with it.  This change allows people to do just that.

```js
markdown: {
  options: {
    template: false
  }
}
```