This PR adds in two upgrades to the current configurable_constants! macro that allows for users to specify the values of configuration constants using environment variables and the like. It adds two things:

• Allows bool values to be parsed by 0, 1, true, false, on, off, etc. configurable_bool_constants! is no longer needed.
• Allows Option to be a specified type with a default value of None, which parses the environment value as type T but puts it in Some(Value) if it's present and None if it's not specified. This allows us to determine if a value has been specified, e.g. in the case where the default depends on other things but can be overridden.