Use loaderUtils.stringifyRequest for relative path (#243)

https://webpack.github.io/docs/how-to-write-a-loader.html#should-not-embed-absolute-paths

Author: cappadona

lib/loader.js

@@ -220,7 +220,7 @@ module.exports = function (content) {
         '  var hotAPI = require("vue-hot-reload-api")\n' +
         '  hotAPI.install(require("vue"), false)\n' +
         '  if (!hotAPI.compatible) return\n' +
-        '  var id = ' + JSON.stringify(filePath) + '\n' +
+        '  var id = ' + loaderUtils.stringifyRequest(loaderContext, filePath) + '\n' +
         '  if (!module.hot.data) {\n' +
         // initial insert
         '    hotAPI.createRecord(id, module.exports)\n' +