Integrate HappyPack to speed up builds

[HappyPack](https://github.com/amireh/happypack) significantly reduces webpack build times by using parallel execution.

I was able to make it work with babel-loader by adding the following to my `backpack.config.js`:
```js
const HappyPack = require('happypack');

module.exports = {
webpack: config => {
let babelOptions;
config.module.rules = config.module.rules.map(rule => {
if (rule.loader === 'babel-loader') {
babelOptions = rule.options;

return {
test: rule.test,
exclude: rule.exclude,
use: {
loader: require.resolve('happypack/loader'),
query: {id: 'babel'},
},
};
}

return rule;
});
config.plugins = [
...config.plugins,
new HappyPack({
id: 'babel',
loaders: [
{
path: require.resolve('babel-loader'),
query: babelOptions,
},
],
verbose: false,
}),
];

return config;
},
};
```

However, it would be great to have this set up by default.

Willing to make a PR if this is something you would want to add.

Start with backpack.config.js and the webpack configuration hook described in the issue, then review the existing babel-loader rule and plugins. Compare the proposed HappyPack integration with the current build setup; done means HappyPack is configured by default and builds continue to process Babel files successfully.

Written by the indexing model from the issue text.