Add AudiobookCovers.com metadata provider

AudiobookCovers.com acts as a cover-only metadata provider, therefore will only show up in the covers selector.

client/store/scanners.js

@@ -63,6 +63,12 @@ export const state = () => ({
       text: 'iTunes',
       value: 'itunes'
     }
+  ],
+  coverOnlyProviders: [
+    {
+      text: 'AudiobookCovers.com',
+      value: 'audiobookcovers'
+    }
   ]
 })