Allow negative durations in conv cmd

[jftuga]

Before this patch, this error would occur when trying to use a negative duration:

$ dtmate conv "-2 hours 3 minutes 4 seconds" ms
Error: unknown shorthand flag: '2' in -2 hours 3 minutes 4 seconds

With this patch:

$ dtmate conv "-2 hours 3 minutes 4 seconds" ms
-123 minutes 4 seconds

Testing

• conv_test.go has been updated to include many negative duration tests

[jftuga]

See comments.