perf: add getCacheKey function

Author: eddyerburgh

jest-vue.js

@@ -1,5 +1,7 @@
 const process = require('./lib/process')
+const getCacheKey = require('./lib/get_cache_key')
 
 module.exports = {
-  process: process
+  process: process,
+  getCacheKey: getCacheKey
 }

lib/get_cache_key.js

@@ -0,0 +1,7 @@
+var crypto = require('crypto')
+
+function getCacheKey (fileData, filename, configString) {
+  return crypto.createHash('md5')
+    .update(fileData + filePath + configString, 'utf8')
+    .digest('hex')
+}