Send all .send elements within active slide to remote in one message

Extract all .send elements and concat them to one. 

Based on this presentation slide;
```
<h3 class="send">Title</h3>
<img some large image/>
<div class="send">continue..</div>
```

The remote will receive 1 message containing;
```
<h3 class="send">Title</h3>
<div class="send">continue..</div>
```