Ability for a custom function using sizes

Hi there 👋

I'm trying to create a plugin that **calculates file sizes of all bundles and then pushes those stats to a custom data store**, Firebase Cloud Firestore in my case. So I've looked into reusing this plugin.

One approach would be to **create a new plugin that extends `SizePlugin`**, overriding the `apply` method to not print to console, and instead use `this.sizes` as I see fit. Somethingl ik

```js
export default class MyCustomSizePlugin extends SizePlugin {
async apply(compiler) {
const outputPath = compiler.options.output.path;
this.output = compiler.options.output;

const afterEmit = (compilation, callback) => {
this.load(outputPath)
.then((sizes)=> {
/* do what I need to do with sizes */
})
.catch(console.error)
.then(callback);
};

/* ... */
}
}
```

_Note: the example above doesn't work fully as expected, still haven't figured out why, but I imagine it's possible to make it work._

Another approach I like more is to **add a new feature to `size-plugin` to offer a custom callback**. This way, we keep pretty printing as well as offer the ability for developers to do any additional operations with the calculated sizes. It could look like this:

```js
// in webpack config
plugins: [
SizePlugin({
onSizesCalculated: (sizes) => { /* custom handling of the bundle sizes */ },
}),
],
```

The code change required would likely be minimal, probably:

```js
if (this.options.onSizesCalculated) {
this.options.onSizesCalculated(Object.assign({}, this.sizes});
}
```

For context, the `this.sizes` object of the plugin looks like this:

```js
{
'bundle.js': 1070,
'1.*****.chunk.js': 102,
'2.*****.chunk.js': 137,
}
```

How do these ideas sound? Am I over-complicating myself and there's a simpler way to solve my problem without using `size-plugin`? Or does the idea of adding a custom callback sound valuable? I'd be happy to open PR if that's the case.