[Bug] No escaping for attributes causes errors

[zisra]

I'm converting HTML to markdown, and when converting images with alt text that has "[" and "]" brackets surrounding it (see example), this fails to escape and fails to render using marked. Should there be a feature to escape the content of HTML attributes as well? Happy to submit a PR to fix this myself.

Example HTML:
<img src="figure.jpg" alt="[Image]">

Leads to:
![[Image]](figure.jpg)

Instead of:
![\[Image\]](figure.jpg)