Add "variables" support

[alessio29]

Let user add "variables":
~set $shoot=s8+2
and use them in rolls
~r $shoot

[dnpetrov]

NB let's call those "macros" to disambiguate them from "variables".

Some mechanics reuse some dice rolls.

Example 1: Dungeon Crawl Classics resolves warrior attacks as follows:

• to-hit: roll d20 + your "deeds die", which grows with your level;
• damage: roll weapon damage, add your "deeds die".

Example 2: (attacks in some dice pool-based system)

• to-hit: roll your to-hit dice pool against to-hit difficulty, count successes
• damage: roll your damage dice pool against damage difficulty modified by to-hit successes

[dnpetrov]

Speaking for macros, it actually seems useful to be able to use macros not only in expressions. E.g.,

!set $init=player A d6 player B d6 bad guys d6
!$init 

(or whatever)