Improve pretty-printing of namespaces

This makes the pretty-printed namespace form more in line with the
clojure style guide.

Specifically we go from:

```
(ns foo.bar
  (:require [this.library :as lib]))
```

To:

```
(ns foo.bar
  (:require
    [this.library :as lib]))
```

This commit closes clojure-emacs/clj-refactor.el#459

src/refactor_nrepl/ns/pprint.clj

@@ -36,7 +36,7 @@
 
 (defn pprint-require-form
   [[_ & libspecs]]
-  (print "(:require ")
+  (print "(:require\n")
   (dorun
    (map-indexed
     (fn [idx libspec]