Fluent interface for ConfigParser

Implement a [Fluent interface](https://en.wikipedia.org/wiki/Fluent_interface) for `ConfigParser`.

As a developer, I want to do
```js
new ConfigParser('...')
.addEngine(...)
.addPlugin(...)
.write();
```

Should be possible as a non-breaking change. If in doubt, we could export something like the following as a first step:
```js
class FluentConfigParser extends ConfigParser {
addPlugin (...args) {
return (super.addPlugin(...args), this);
}
addEngine (...args) {
return (super.addEngine(...args), this);
}
}
```

Start by locating ConfigParser and its addEngine, addPlugin, and write methods. Check how existing calls behave, then verify that the fluent example chains these methods without breaking the current API.

Written by the indexing model from the issue text.