Problem with spans across lines.

[ralsina]

Hi! I am using this plugin to get line numbers in my book code samples, and I ran into an issue.

When faced with this kind of code:

```python
"""Usage:

whatever
"""
```

Markdown will be converted into this (by the highlight plugin?):

<span class="hljs-string">"""
Usage:
whatever
""""</span>

And then when your plugin tries to wrap lines in code-line spans, things break because opening/closing tags don't match anymore:

<span class="code-line"><span class="hljs-string">"""</span>
<span class="code-line">Usage:</span>
<span class="code-line">whatever</span>
<span class="code-line">""""</span></span>

The effect is that the whole thing is only counted as one line.

It doesn't look easy to fix, it would need to keep track of open spans and close them then reopen them in the next line?