@repl output rendering woes

[thofma]

Thanks for #104! I did some more testing and noticed that DV is confused when output is suppressed with ; or contains #. For example

```@repl
1 + 1;
1 + 2;
[1, 2]
1 + 3
```

```@repl
1 + 1; # hide
1 + 2;
[1, 2]
1 + 4
```

gets rendered as

Note that there are superfluous newlines at the beginning and missing newlines in between.

The HTML writer handles this (a bit) better:

[thofma]

Fixed by #111