support puppetInstance() to save/load instance of puppet

src/config.js

@@ -71,4 +71,14 @@ function isWechatyDocker() {
   return true
 }
 
+/**
+ * 5. live setting
+ */
+Config.puppetInstance = function(instance) {
+  if (typeof instance !== 'undefined') {  // null is valid here
+    Config._puppetInstance = instance
+  }
+  return Config._puppetInstance
+}
+
 module.exports = Config.default = Config.Config = Config

src/config.spec.js

@@ -35,12 +35,20 @@ test('Config methods', t => {
       Config.validApiHost(apihost)
     })
   }, 'should not row for right apihost')
-  ERR_APIHOSTS.forEach(apihost => { 
+  ERR_APIHOSTS.forEach(apihost => {
     t.throws(_ => {
       Config.validApiHost(apihost)
     })
   }, 'should throw for error apihost')
 
   t.true('isDocker' in Config, 'should identify docker env by `isDocker`')
+})
 
-})
\ No newline at end of file
+test('Config puppetInstance', t => {
+  let instance = Config.puppetInstance()
+  t.falsy(instance, 'should empty initialy')
+  t.truthy(Config.puppetInstance({}), 'should return instance in arg')
+  t.truthy(Config.puppetInstance(), 'should return instance after set')
+  t.falsy(Config.puppetInstance(null), 'should return null after set to null')
+  t.falsy(instance, 'should empty after set to null')
+})