groupstats.pl: Add '-f' option.

Add '-f' switch to save monthly reports to a file with
a filename made up by adding year and month to template
submitted via '-f filename': filename-YYYY-MM
Modify OutputData() accordingly.
Ignore '-c' if '-f' is set.
Allow & force '-o dump' if '-f' is set even if a time
period is defined.
Change documentation accodingly.

Fixes #27.

Signed-off-by: Thomas Hochstein <thh@inter.net>