Idiomatic `get` usage

I see only a one problem with my version - when user explicitly specifies a `nil` value in config, then defaults will not be used:

```clojure
user=> (def defaults {:a 1 :b 2})
#'user/defaults
user=> (def loaded-config {:a 100 :b nil})
#'user/loaded-config
user=> (get loaded-config :a "DEFAULT")
100
user=> (get loaded-config :c "DEFAULT")
"DEFAULT"
user=> (get loaded-config :b "DEFAULT")
nil
```

Thoughts?