Please add default width option

First of all, thanks for building a great plugin!
On one of my sites I found several errors in console regarding the srcset w-descriptor.

Thanks to a post by @jodamo5 I was able to solve the error by adding a default width to SVG images. I think that adding the code by @jodama5 would be a nice addition to your plugin which could help users.

The post from @jodamo5:
https://wordpress.org/support/topic/failed-parsing-srcset-attribute-value-since-its-w-descriptor-is-invalid/#post-7681499