Removed erroneous "i -= 1" while merging page numbers down

Just after a del self.wb[i] when finding two .pn commands in a row
ppgen had
  i -= 1  counteract i += 1 below
  continue
The decrement is unneeded because with the "continue" it never reaches
the increment operation. In practice, the decrement can cause the page
number to actually move up a line into text rather than down as intended.
retain page numbers in .nf b/l/r blocks

ppgen was losing page numbers in .nf b/l/r blocks because a call
to re.sub replaced all leading tags or page numbers instead of
capturing each and replacing them one by one.
Merge pull request #52 from wf49670/develop

Removed erroneous "i -= 1" while merging page numbers down