Adjacent code blocks get merged

[egrimley]

Adjacent code blocks get merged:

$ printf '```\naa\n```\n```\nbb\n```' | cmark -t html
<pre><code>aa
</code></pre>
<pre><code>bb
</code></pre>
$ printf '```\naa\n```\n```\nbb\n```' | cmark -t commonmark | cmark -t html
<pre><code>aa

bb
</code></pre>

Parenthetical remark: This doesn't happen with single-character code blocks because of the logic at commonmark.c:294:

$ printf '```\n1\n```' | cmark -t commonmark
``` 
1
```
$ printf '```\n12\n```' | cmark -t commonmark
    12

An easy way to fix this would be to delete about 10 lines from commonmark.c and render all code blocks without indentation.

Personally I prefer not to use indentation for code blocks. One could argue that it's better because it lets one copy-paste the code more easily. I'm not using indentation in this text box.

[jgm]

I agree, it probably makes sense to switch to using fenced code blocks in the commonmark renderer.