Add line break after code blocks

All block-level elements have line break after close tag, but
indent-style code blocks does not. This bug appeared on [e3692c772b][1].

Input:

        Some pre formatted text.
        Other pre formatted text.
        Another pre formatted text.

    Normal paragraph.

Output:

    <pre><code>Some pre formatted text.
    Other pre formatted text.
    Another pre formatted text.</code></pre><p>Normal paragraph.</p>

[1]: https://github.com/chjj/marked/commit/e3692c772be8a8e12b53319baa7643fbd37ae666