Copying order/unordered list in view mode don't preserve the format

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

Can not copy full order/unordered list in view mode after selection of text.
Ordered List item numbers and Unordered list item dashes are dropped, list formats are not preserved when pasting.

#### Current behaviour

Provide steps to reproduce:

1. Select a list in view mode
2. Ctrl - C
3. Ctrl - P
4. Format of the list is lost.

#### Expected behaviour

Copying list in view mode should act like copying from edit mode.
- Unordered list should use dashes as prefix, and preserve the hierarchical structure.
- Ordered list should be able to copy along with the rendered item number.

#### Environment

I use HackMD on:

**Desktop**
- OS: Manjaro Linux
- Browser: Brave
- Browser Version: 1.57.57