[suggestion] add option to change target attribute name

Hi!

An option to change the target attribute name would come in handy, for example with html5 banners exported from Google Web Designer:
`<img is="gwd-image" source="background.jpg" id="background1" class="gwd-img-332h gwd-gen-8pp1gwdanimation">`
These have no src attribute, and use "source=" instead. I imagine this could come in handy in other circumstances too.
