feat(store): default to memory store

lib/index.js

@@ -2,15 +2,16 @@
 
 import readCache from './read-cache'
 import serialize from './serialize'
+import memoryStore from './memory'
 
 function cache (config = {}) {
-  if (!config.store) {
+  const store = config.store || memoryStore
+  const key = config.key || cache.key
+
+  if (!store) {
     throw new Error('Cache middleware need to be provided a store.')
   }
 
-  const store = config.store
-  const key = config.key || cache.key
-
   config.maxAge = config.maxAge || 0
   config.readCache = config.readCache || readCache
   config.serialize = config.serialize || serialize