textfill: get prefix from regexp when appropriate.

textfill: get prefix from regexp when appropriate.

Revise code for finding start of para so that if the regexp identifies a
prefix - rather than a whole line - we use that prefix to decide where
para starts, rather than skipping punctuation.

Also include trailing spaces of a prefix in the prefix for .md files.

Signed-off-by: NeilBrown <neil@brown.name>