Allow for passing negative arguments to -d
Allow for passing negative arguments to -d

When specifying date ranges using -d, allow for passing negative values
to indicate that the date range should start a certain number of days
ago.

Signed-off-by: Lukas Fleischer <c9ca73f8336dcfb037ec2c2abf52d80a6847a5cd@calcurse.org>

lfos/calcurse,lfos/calcurse,lfos/calcurse