<iframe> tag cause open redirect

If the source website has the script like this:
```

if(window != top) {
top.location.href = location.href;
}

```
It may cause a open redirect issue on codimd.
I use `www.plurk.com` which has anti-clickjacking code to demo.

[Demo Link in demo.codimd.org](https://demo.codimd.org/I_H9aum-Tb-kbBDpXmPqow#)
```

```

Broswer verison:
```
Safari 11.0.2: triggered
Firefox Quantum 62.0 : triggered
Chrome 68.0.3440.106: not triggered
```