update preformatted syntax mode to ignore entire blocks of white space

This patch only updates the actual "preformatted" syntax mode, that is
only preformatted blocks caused by leading blanks on a line.  If the entire
block is empty, it is ignored.  ie. if a block of preformatted text has
leading or trailing empty lines and some non-empty lines, the leading and
trailing content are preserved.

darcs-hash:20080313203950-f07c6-9d0745b157e45b61b06ff0c7d85bec8558946c1e.gz