config-overrides.js does not support async

[ooooevan]

I want to do something async before module.exports, but it does not support
related to #365

module.exports = function override(config, env) {
  return new Promise((resolve)=>{
    // code...
    resolve(config)
  })
};

Failed to compile.
Entry module not found: Error: Cant resolve './src' in 'E:\xxx\xx'

can it supported async ?

version: ^2.1.3

[patricktran]

I just submitted a PR to add async/wait support for devServer configuration
#543