CLJS-2423: Allow custom :output-wrapper function

Expands support from 2 types to 4 types:
- function
- string (`format` compatible string interpolation)
- truthy (that is of course, neither function nor string)
- falsey

Previously, any truthy value would cause the javascript to be wrapped by some
default stringified function. For most simple applications, this would have
sufficed. In cases where the `this` context in the wrapper function is used,
something more expressive is needed to be able to assign this context.
The string and function support aims to fill this gap.

src/main/clojure/cljs/closure.clj

@@ -1869,8 +1869,11 @@
 
 (defn add-wrapper [{:keys [output-wrapper] :as opts} js]
   (if output-wrapper
-   (str ";(function(){\n" js "\n})();\n")
-   js))
+    (cond
+      (fn? output-wrapper)     (output-wrapper js)
+      (string? output-wrapper) (format output-wrapper js)
+      :else                    (str ";(function(){\n" js "\n})();\n"))
+    js))
 
 (defn add-source-map-link [{:keys [source-map output-to] :as opts} js]
   (if source-map