Reuse constants from application in documentation

[StefSchenkelaars]

I would like to be able to reuse definitions from the application in the API documentation without specifying them a second time. So to solve this i've made it possible to pass a constant that will be evaluated.

# @model
# @property currency [enum<{Product::CURRENCIES}>] The currency of the product
class Product
  CURRENCIES = %w[usd eur]
end

[nicksieger]

I love this idea! Great implementation and test cases too. Thanks for the contribution!