Formatting is removed, when copying rich text

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io

### What's the problem

#### Current behaviour

1. View Document in View or Split Mode
2. Copy text
3. Paste in other editor like Word

It got plain text.

#### Expected behaviour

Formatting is copied as everywhere on the web

#### Environment

I use HackMD on:

**Desktop**
- OS: Windows
- Browser: Chrome

- Device: [e.g. iPhone XS]
- OS: [e.g. iOS 12.1]
- Browser: [e.g. chrome, safari]
- Version: [e.g. 23]

#### Additional context

The Copy event is catched.
a.clipboardData.setData("text/plain", window.getSelection().toString().replace(/\u200b/g, ""))