feat: support run single test

test/unit/karma.conf.js

@@ -4,7 +4,7 @@ require('babel-core/register')({
   presets: [require('babel-preset-env')]
 });
 
-var webpackConfig = require('./get-webpack-conf');
+var getWebpackConfig = require('./get-webpack-conf');
 var travis = process.env.TRAVIS;
 
 module.exports = function(config) {
@@ -16,7 +16,7 @@ module.exports = function(config) {
     preprocessors: {
       './index.js': ['webpack', 'sourcemap']
     },
-    webpack: webpackConfig,
+    webpack: getWebpackConfig(getTestFileName()),
     webpackMiddleware: {
       noInfo: true
     },
@@ -30,3 +30,8 @@ module.exports = function(config) {
     singleRun: false
   });
 };
+
+function getTestFileName() {
+  const flagIndex = process.argv.indexOf('--file');
+  return flagIndex !== -1 ? process.argv[flagIndex + 1] : '';
+}