load-config src-spec syntax for targeting a specific environment

Make it possible to load data from a source that does not internally specify environment(s)

``` clojure
(load-config
  {:target :dev :src :lein :as :environ}
  {:target [:staging :prod] :src args :as :cli}  ; args from command-line in -main
  {:target [:staging :prod] :src :sys :as :environ})
```
