Support assigning % dimensions to all width/height targets

Hi there,

Thanks for your work on this, it's a great time-saving tool for processing stuff for mailing!

I noticed that you've restricted the width and height dimensions being applied to only `px`, except for tables which are allowed to have a `%` value. Would you consider supporting `%` for both or is there a specific reason it's this way? I have a use case where I want to stretch an image to 100% wide of the parent container.

Start by locating the width and height handling described in the issue, especially the existing exception that permits percentage values for tables. The work is done when percentage dimensions are accepted for all width and height targets, including an image set to 100% of its parent container.

Written by the indexing model from the issue text.