Add options to specify headers and footers for timing report

- Headers and footers surround the stats for each individual test.  The
  customizability can aid in formatting, making reading easier either
  for humans or for programs.
- The Timer class has been updated accordingly, to allow for setting and
  using a header and a footer.
- The short options for --max-cpu and --max-real have been changed, to use
  -r as the short option for --header.
- The strings supplied with the options are un-escaped, to allow for
  specification of newlines, tabs, etc.