Add filter rime::Traditionalizer

A new filter ```rime::Traditionalizer``` has been added. When enabled, all Simplified Chinese characters/phrases will be converted to Traditional Chinese.

I understand that excluding all Simplified Chinese characters/phrases from Traditional Chinese does not make much sense. However, having Simplified Chinese characters/phrases mixed with Traditional Chinese in composing candidates is indeed inconvenient when trying to learn Traditional Chinese, especially for those people like me, who used to use Simplified Chinese all the time and eventually realized that Traditional Chinese is actually the preferred Chinese.

Corresponding changes have been made on Squirrel, you could probably fetch and merge these commits from the ```master``` branch of [my fork of Squirrel repository](https://github.com/lembacon/squirrel).
(Since I'm somehow too lazy to open another pull request on the repository of Squirrel.)

This whole idea might be terrible, but I'm looking forward to your opinions. I myself find this quite useful, since I'm currently trying to learn to use Traditional Chinese (a.k.a 正體字).

To ensure these changes won't break existing users' experience, I have not changed any of those default settings. In order to use this filter, users should patch their own ```<schema>.custom.yaml``` and ```default.custom.yaml```.

 For example, I am using ```double_pinyin_mspy```, so I have:

```double_pinyin_mspy.custom.yaml``` looks like this:

```yaml
patch:
  engine/filters:
    - traditionalizer
    - uniquifier
  switches:
    - name: ascii_mode
      reset: 0
      states:
        - "中文"
        - "西文"
    - name: full_shape
      states:
        - "半角"
        - "全角"
    - name: traditionalizing
      states:
        - "汉字"
        - "漢字"
```

and, ```default.custom.yaml``` looks like this:

```yaml
patch:
  switcher/save_options:
    - full_shape
    - traditionalizing
    - extended_charset
```