fix warning breaking on windows path (fix #145)

Author: yyx990803

lib/loader.js

@@ -173,8 +173,10 @@ module.exports = function (content) {
         'if (__vue_script__ &&\n' +
         '    __vue_script__.__esModule &&\n' +
         '    Object.keys(__vue_script__).length > 1) {\n' +
-        '  console.warn("[vue-loader] ' + path.relative(process.cwd(), filePath) +
-          ': named exports in *.vue files are ignored.")' +
+        '  console.warn(' + JSON.stringify(
+            '[vue-loader] ' + path.relative(process.cwd(), filePath) +
+            ': named exports in *.vue files are ignored.'
+          ) + ')' +
         '}\n'
     }
   }