Fix Dependency Issues

- Use peer dependencies to state relationship with hexo
- Re-install `level` required package (can be used to create local S3 cache for performance, see [s3-sync](https://github.com/hughsk/s3-sync))

package.json

@@ -21,11 +21,12 @@
   "license": {
     "type": "MIT"
   },
-  "engines": {
-    "hexo": ">= 3.0.0"
+  "peerDependencies": {
+    "hexo": "3.x"
   },
   "dependencies": {
     "chalk": "^1.0.0",
+    "level": "^0.18.0",
     "readdirp": "^0.3.3",
     "s3-sync": "^0.5.1"
   }