browser: expose storage

This patch allows overriding the default `storage` mechanism.

Closes #188.

browser.js

@@ -11,17 +11,10 @@ exports.formatArgs = formatArgs;
 exports.save = save;
 exports.load = load;
 exports.useColors = useColors;
-
-/**
- * Use chrome.storage.local if we are in an app
- */
-
-var storage;
-
-if (typeof chrome !== 'undefined' && typeof chrome.storage !== 'undefined')
-  storage = chrome.storage.local;
-else
-  storage = localstorage();
+exports.storage = 'undefined' != typeof chrome
+               && 'undefined' != typeof chrome.storage
+                  ? chrome.storage.local
+                  : localstorage();
 
 /**
  * Colors.
@@ -129,9 +122,9 @@ function log() {
 function save(namespaces) {
   try {
     if (null == namespaces) {
-      storage.removeItem('debug');
+      exports.storage.removeItem('debug');
     } else {
-      storage.debug = namespaces;
+      exports.storage.debug = namespaces;
     }
   } catch(e) {}
 }
@@ -146,7 +139,7 @@ function save(namespaces) {
 function load() {
   var r;
   try {
-    r = storage.debug;
+    r = exports.storage.debug;
   } catch(e) {}
   return r;
 }