Optional Enumerables do not create empty instances when omitted

[bmurphy23]

This PR address #15. If an Enumerable attribute (map or list-of-maps) is flagged as optional, and that attribute is not in the input, I'd expect the output from Speck.validate/2 to be as minimal as possible. For a top-level attribute, that means a nil value. For anything below the top level, that would mean being entirely omitted from the output. To make this happen, I've made the handling of optional values a higher priority in apply_filters.

One behavioural change not directly related to the above is that a non-optional Enumerable that is omitted will not recursively report :not_found errors. Instead, the map/list-of-maps will itself be marked as :not_found, and the interior elements skipped entirely. The old behaviour is easy enough to re-implement by moving a condition in apply_filters.

[bmurphy23]

After more exhaustive testing, there's still some behaviour that seems not quite right afoot. I'm tinkering with the code more.