Extra newlines between HTML blocks should be preserved

stupicat turns this:

```

foo

let bar;

let baz;

```

into this:

```

foo

let bar;

let baz;

```

But apparently without the extra newline, pulldown-cmark doesn't think the `` tag is its own block:

```
$ pulldown-cmark <foo.md
<p>
foo
</p>
<script>
let bar;

let baz;

$ cargo run --example stupicat -- foo.md | pulldown-cmark
Finished dev [unoptimized + debuginfo] target(s) in 0.02s
Running `target/debug/examples/stupicat foo.md`

foo

let bar;
<p>let baz;

```

Obviously the tags in the middle of the `` mangle everything.