Unfiltered class attribute in markdown code

Hello, CodiMD:

This bug affects every note in CodiMD.

It could be used to create a phishing note.

I have not reviewed the code about this, however I suggest to filter every class attributes.

Here's the PoC code:
### POC
```

```
And here's the demo phishing page: [Demo Note](https://hackmd.io/TFu9gvm6R7O2rwdEAKdlzg#)


### Suggested fix
* Filter class attributes

Ref: https://github.com/ChALkeR/notes/blob/master/Improper-markup-sanitization.md