refactor: initPlatform

lib/platform/index.js

-const github = require('./github');
-const gitlab = require('./gitlab');
-const vsts = require('./vsts');
+/* eslint-disable global-require */
+const platforms = new Map([
+  ['github', require('./github')],
+  ['gitlab', require('./gitlab')],
+  ['vsts', require('./vsts')],
+]);
+/* eslint-enable global-require */
 
-function initPlatform(val) {
-  if (val === 'github') {
-    global.platform = github;
-  } else if (val === 'gitlab') {
-    global.platform = gitlab;
-  } else if (val === 'vsts') {
-    global.platform = vsts;
-  }
+function getPlatformApi(platform) {
+  return platforms.get(platform);
+}
+
+function initPlatform(platform) {
+  global.platform = getPlatformApi(platform);
 }
 
 module.exports = {
   initPlatform,
+  getPlatformApi,
 };