Move max level cap into config

[arbron]

Currently, the max level cap of 20 is hardcoded as a magic number in a number of places throughout the codebase. This makes it difficult to find where the level cap is used and very difficult for any module that wants to modify the level cap in 5e (e.g. any module that wants to implement post-level-20 content).

I propose adding a new constant into the config file DND5E.levelCap which defaults to 20 to allow for a single point of customization and more self-documenting code.

Additional Idea

One other idea is also adding a DND5E.classLevelCap which is a separate cap for classes to allow characters and classes to be capped at a different level (e.g. if you wanted classes to max out at level 20, but allow multi-classing for players to higher levels past that).

Alternative Naming

DND5E.maxLevel (+ DND5E.maxClassLevel)