Add support for image titles

[nvanexan]

Adding support for image titles per CommonMark Spec: https://spec.commonmark.org/0.30/#example-571

This can be used downstream to wrap images with <figure> elements and with <figcaption> elements for the title where the attributes return a title.

Example:

![foo](/url "title")

<figure>
   <img src="/url" alt="foo" />
   <figcaption>title</figcaption>
</figure>

[rpaul-stripe]

Thanks for contributing! This looks good to me. 👍