feature: Support srcset for img

[marcstern]

Same request than #5 with srcset:
<img decoding="async" loading="lazy" src="images/pentest-report-download-image-01-209x300.png" alt="" class="wp-image-109" width="270" height="387" srcset="https://xxx/wp-content/uploads/2022/09/Pentest-Report-Download-Image-01-209x300.png 209w, https://xxx/wp-content/uploads/2022/09/Pentest-Report-Download-Image-01-715x1024.png 715w" sizes="(max-width: 270px) 100vw, 270px" />

I added "srcset" the same way but it doesn't seem suffisant. Did I miss something?
I guess we should add it anyway (and potentially fix the parser if needed).