Add env? to check for the existence of a value

I often find myself adding this utility function to projects that use environ:

```clj
(defn env? [k]
  (not (str/blank? (env key))))
```

`(if (env? :key) ...)` is preferable to `(if (env :key) ...)` because it allows config to be _removed_ by system properties (`-Dkey=""`) or environment variables (`KEY=''`) not just added.

Would you be interested in adding this function to the core? If not, feel free to close this as I understand if you prefer to keep this outside the library.