htmlfill.render: Ignore <option> inside <datalist>

In HTML5, `<option>` is valid within `<datalist>` as well as `<select>`, but *FormEncode* considers this a fatal error. This patch adjusts `htmlfill.render` to ignore `<option>` elements that it finds within `<datalist>`, since they are purely for client-side purposes and have no value to form validation.