Persist attributes defined on code blocks

[joepavitt]

When using https://www.npmjs.com/package/markdown-it-attrs we are able to define attributes to be appended to elements in our markdown, e.g:

paragraph {data-toggle=modal}

would render:

<p data-toggle="modal">paragraph</p>

It does also support injection of attributes on the fenced code blocks, but this Eleventy Plugin does not consider the attrs defined. This PR adds that in such that the introduced parent <div> takes ownership of the attributes.

For example, if, in my markdown, I include:

```python
nums = [x for x in range(10)]
```

```python {data=hello}
nums = [x for x in range(10)]
```

Then the following elements are rendered to my DOM:

[joepavitt]

This does also include a change from #12 - happy to revert that in favour of this attrs approach if that's preferred.

[kudoh]

Thanks a lot for the PR! This is really helpful.
I’ve reviewed the changes, and everything looks good, so I don’t think we need to revert anything.
I’ll be adding some unit tests after merging and bumping up the version afterward.

[joepavitt]

Thanks for such a quick turnaround @kudoh