Dice: Add advantage/disadvantage syntax

[derikb]

Some kind of dice notation to represent situations like this:

• Roll x take highest.
• Roll x take lowest.
• Roll x drop lowest.
• Roll x drop lowest.

[derikb]

Looking around for syntax options... this is what Roll20 uses and it seems clear/simple:

• dh drop highest x dice
• dl drop lowest x dice
• kh keep highest x dice
• kl keep lowest x dice

So a 5e disadvantage would be 2d20kl1 or 2d20dh1 and an advantage would be 2d20kh1 or 2d20dl1.

Not super big fan of that d in there for drop since it complicates the syntax parsing since we also use d for dice.