feat: Expect plugins to return Promises

BREAKING CHANGE: Each plugin is expected to return an async function or a Promise returning function. The callback parameter is not passed to plugins anymore.

lib/plugins/normalize.js

@@ -1,4 +1,4 @@
-const {promisify, inspect} = require('util');
+const {inspect} = require('util');
 const {isString, isObject, isFunction, noop, cloneDeep} = require('lodash');
 const importFrom = require('import-from');
 
@@ -14,9 +14,9 @@ module.exports = (pluginType, pluginConfig, logger, validator) => {
 
   let func;
   if (isFunction(plugin)) {
-    func = promisify(plugin.bind(null, cloneDeep(config)));
+    func = plugin.bind(null, cloneDeep(config));
   } else if (isObject(plugin) && plugin[pluginType] && isFunction(plugin[pluginType])) {
-    func = promisify(plugin[pluginType].bind(null, cloneDeep(config)));
+    func = plugin[pluginType].bind(null, cloneDeep(config));
   } else {
     throw new Error(
       `The ${pluginType} plugin must be a function, or an object with a function in the property ${pluginType}.`