Enable reference-style for convert plugin

### Convert plugin reference-style
You can define refer-id for plugins as links or images.


```
::: [refer-id-1] :::

::: [refer-id-2
contents of plugin
:::

** Reference Definition **
[refer-id-1]: plugin param, param
[refer-id-2]: plugin param
```

### Defining referred plugin parameters like Yaml front-matter

```
[refer-id]: plugin
---
foo: bar
hoge: fuga
---
```