When webpack is configured as multi compile. the browser is opened before the first emit

As the title states when you're running webpack in multi-compiler mode e.g.

```
module.exports = [
{...},
{...},
]
```

this plugin fires straight away, which results in a page without any bundles to inject.
This might be pretty edge case, but it would be cool for us who have multiple configurations for running es5 and es2015+ simultaneously.

