Declarative way for compute facts

Hi! I consider this repo as a very helpful design approach! I'm looking forward and have thoughts about admin interface for editing and storing rules. But i think we have a lack of ability for storing complex computed facts. For example, i have a a big legacy system underneath my app, and some facts from there have a big value because they computes from multiple fields of big json document. I need to reuse that facts everywhere in my app. What can i do right now? I can use only functions for this objectives. Like this:

```
engine.addFact('account', (params, almanac) => {
return almanac.bigDoc.path1.value + almanac.bigDoc.path2.value;
})
```

Ok, it's very flex and this approach cover all my needs, except storing.

My suggest:

We can use [jsonLogic library ](https://github.com/jwadhams/json-logic-js/) (or similar operators inside this repo) for declaring facts like this:

```
new Fact('derivedFact', [ 'bigDoc' ], { '+' : [
{ 'var' : 'bigDoc.path1.value' },
{ 'var' : 'bigDoc.path2.value' },
]}
)
```

While i was browsing docs, i saw that you considered this as a design bad smell. Why? Can you explain? Or i can suggest PR? 🙂

未指定來源檔案或測試。先檢視文件中對 computed facts 的討論以及現有的 addFact/function 方法，然後比較針對可重複使用的衍生 facts 所提出的 json-logic 宣告。只有在 maintainer 決定這是否屬於 engine，並且在接受的情況下形成一份範圍明確的儲存與評估設計後，才算完成。

由索引模型根據 Issue 內容生成。