Support adding constants to dice expressions

[brentleyjones]

A common use case for dice notation is the ability to add constants to the result of dice rolls. Expressions (see #3) should be extended to support addition of constants to support this.

To get this in one go we should support the concept of a "named constant". An example of this is 2d6 + 1d4 per level (or 2d6 + (LEVEL)d4), where level evaluates to the same constant wherever it's used in the expression. (Moved to #25)