Async webpack config

Nobody has claimed this yet.

I need to do some async work to create the webpack config. According to [webpack's docs](https://webpack.js.org/configuration/configuration-types/#exporting-a-promise), you can export a function that returns a promise to handle this.

However when I try to do a similar thing with backpack, I get an error.

Defining the webpack config as a function that returns a promise (or as an async function):
```javascript
module.exports = {
webpack: (config) => {
return new Promise((resolve, reject) => {

// config changes here

resolve(config);
});
}
};
```
results in:
```
WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
- configuration misses the property 'entry'.
object { : non-empty string | [non-empty string] } | non-empty string | [non-empty string] | function
The entry point(s) of the compilation.
at webpack (/Users/me/repos/project/node_modules/webpack/lib/webpack.js:19:9)
at Object. (/Users/me/repos/project/node_modules/backpack-core/bin/dev:28:24)
at Module._compile (module.js:635:30)
at Object.Module._extensions..js (module.js:646:10)
at Module.load (module.js:554:32)
at tryModuleLoad (module.js:497:12)
at Function.Module._load (module.js:489:3)
at Function.Module.runMain (module.js:676:10)

at startup (bootstrap_node.js:187:16)
at bootstrap_node.js:608:3
```

Any chance that async configuration could be added to backpack as well?

Start at backpack-core/bin/dev, the entry point named in the stack trace, and compare its webpack configuration handling with webpack's documented promise-export behavior. Reproduce the reported async configuration and consider the work complete when it is accepted without the missing-entry validation error.

Written by the indexing model from the issue text.