use hash-sum for module id generation (vuejs#160)

Author: gzzhanghao

lib/gen-id.js

@@ -1,8 +1,8 @@
 // utility for generating a uid for each component file
 // used in scoped CSS rewriting
-var fileUid = 1
-var fileRegistry = Object.create(null)
+var hash = require('hash-sum')
+var cache = Object.create(null)
 
 module.exports = function genId (file) {
-  return fileRegistry[file] || (fileRegistry[file] = fileUid++)
+  return cache[file] || (cache[file] = hash(file))
 }