Hello,

I am a grateful and happy user of the new shr-fill-text option; one nit
I have with its current implementation is that, since setting it to nil
essentially short-circuits shr-fill-lines, we skip shr-fill-line, which
actually handles both filling *and* indentation.

This is very noticeable when reading HTML emails with Gnus: citations in
blockquotes become indistinguishable from the actual reply.  See
eww-current.png for a demonstration with EWW: 

* left is shr-fill-text t (default),
* right is shr-fill-text nil and visual-line-fringe-indicators
  '(left-curly-arrow right-curly-arrow).

The attached patch makes it so that shr applies indentation before it
consult the shr-fill-text option and skips filling.  See eww-patch.png
for a demonstration: 

* left is shr-fill-text nil and visual-line-fringe-indicators
  '(left-curly-arrow right-curly-arrow),
* right is the same, plus visual-wrap-prefix-mode,
* not pictured (unchanged): shr-fill-text t (default).

This patch also adds a rendering testcase, and teaches the rendering
test to re-run testcases while setting extra user options to make sure
they do not impact rendering.

Curious to hear your thoughts.  Will post an updated patch once I have a
bug number to replace the "bug#TODO" placeholders.

Thanks!