Chainable Options 
by Anthony Fu @antfu
Chainable options are commonly used in Javascript. But when we switch to TypeScript, can you properly type it?
In this challenge, you need to type an object or a class - whatever you like - to provide two function `option(key, value)` and `get()`. In `option`, you can extend the current config type by the given key and value. We should about to access the final result via `get`.
For example
```ts
declare const config: Chainable
const result = config
.option('foo', 123)
.option('name', 'type-challenges')
.option('bar', { value: 'Hello World' })
.get()
// expect the type of result to be:
interface Result {
foo: number
name: string
bar: {
value: string
}
}
```
You don't need to write any js/ts logic to handle the problem - just in type level.
You can assume that `key` only accepts `string` and the `value` can be anything - just leave it as-is. Same `key` won't be passed twice.
