add support :skip-goog-provide in ns meta

In cljsjs shims for firebase the (ns firebase.app) declaration
was clashing with the firebase var it tries to export. The ns
is unused otherwise anyway so we can just skip emitting goog.provide

project.clj

@@ -82,7 +82,7 @@
 
    ;; experimental
    [hawk "0.2.11"]
-   [thheller/shadow-cljsjs "0.0.18"]]
+   [thheller/shadow-cljsjs "0.0.19"]]
 
   :source-paths
   ["src/main"]

src/main/shadow/build/compiler.clj

@@ -142,7 +142,6 @@
           ))))
   ast)
 
-
 (defn find-js-require-pass [env ast opts]
   (when (and (= :invoke (:op ast))
              ;; js/require called at the REPL should not be added to metadata since it becomes sticky
@@ -289,7 +288,9 @@
   ;; FIXME: can't remove goog.require/goog.provide from goog sources easily
   ;; keeping them for CLJS for now although they are not needed in JS mode
   #_(when (= :goog (get-in state [:build-options :module-format])))
-  (comp/emitln "goog.provide('" (comp/munge name) "');")
+
+  (when-not (get-in ast [:meta :skip-goog-provide])
+    (comp/emitln "goog.provide('" (comp/munge name) "');"))
 
   (when (= name 'cljs.js)
     ;; this fixes the issue that cljs.js unconditionally attempts to load cljs.core$macros