Suggestion: Unset keys should be falsy

Nobody has claimed this yet.

I love configatron, but although it solves a lot of things, it makes it rather difficult to work with unassigned keys.

In a standard hash, or an `OpenStruct`, non existing values are falsy:

```ruby
o = OpenStruct.new({ one: '1', two: '2' })
p o.three
# => nil
```

While in configatron they are truthy:

```ruby
# configatron.allow = false # unset
p configatron.allow ? "allowed" : "not allowed"
# => "allowed" (but expected "not allowed")
```

Even if I want to set defaults, I cannot work with the common ruby syntax:
```ruby
configatron.allow ||= false
```

And instead, I can only do:
```ruby
configatron.allow = false unless configatron.has_key? :allow
```

And in cases where I can't (or prefer to avoid) having defaults, in order to allow "open schema", the only way for me to handle unset keys in the same way as falsy keys, is:

```ruby
allowed = configatron.has_key?(:allow) ? configatron.allow : false
p allowed ? "allowed" : "not allowed"
```

Am I missing some functionality in configatron that will make this easier?

If there is no way to alleviate the above issues, I would love to see a change that allows it - even if it will require me to opt-in to this behavior using some config directive (in case it is desired to avoid introducing a breaking change).

No file, test, or entry point is named. Start by tracing how configatron handles reads of unset keys, then compare that behavior with the issue's OpenStruct and ||= examples; done means unset keys behave as falsy without breaking existing configured values, with any opt-in requirement resolved.

Written by the indexing model from the issue text.