Request: Add image URLs to link reference definitions

[NfNitLoop]

This is sortof a follow up to #15. I'm still using this library years later! 😊

Link reference definitions are working great, except for one cumbersome edge case, images.

Given this input:

<a href="VERY_LONG_URL"><img src="VERY_LONG_URL"/></a>

With useLinkReferenceDefinitions enabled, we get this output:

[![](VERY_LONG_URL)][1]

[1]: VERY_LONG_URL

The VERY_LONG_URL is duplicated, when we could have instead used a link reference for both:

[![][1]][1]

[1]: VERY_LONG_URL

[NfNitLoop]

If you're open to a PR w/ this change, let me know!