Here's a patch that implements the "wrap-prefix" feature discussed
earlier.  It allows a prefix to be added to the beginning of each line
wrapped by redisplay.  It works with both word-wrapping and non-word
wrapping.

I also added a "line-prefix" feature which is the same thing for _non_
wrapped lines.

There are both buffer-local variables (`line-prefix', `wrap-prefix') and
text-property/overlay versions of both.

They may contain a string, an image or a `display'-property style
"space" spec.

Besides my current usage (adding "hanging indents" to word-wrapped-lines
in rcirc buffers), these features could be used to add other sorts of
word-processor-style spacing frobs, such as an implicit paragraph indent
(using line-prefix), additional vertical whitespace between "paragraphs"
(using a line-prefix with a value like (space :height 1.5)), etc.

Any comments?

-Miles