Fix positive signs in dString

`rolldice +3 +6` was valid, but `rolldice +3D+6` wasn't. This should fix that
inconsistency.
Add unique rolls implementation

This provides an alternative way of rolling dice which guarantees no two dice
will have the same value. Obviously the algorithm is based on a shuffle of all
possible die faces.

Also corrects minor capitalization error in doc comment of roll.
Improved rollUnique

Rather than shuffling all d numbers, we can just shuffle until we have n
random numbers, and then carry on. Useful if n is much smaller than d.
Add unique rolls feature

Bump to v1.4.0