relative image reference in HTML is broken

[Jerakin]

Description

HTML with relative image doesn't render.

I tried this to get it to render.

<p align="center">
<img src="./images/start.png">
</p>

Markdown way works fine ![start](./images/start.png)

Using absolute images works too

<p align="center">
<img src="https://raw.githubusercontent.com/marktext/marktext/master/static/logo-small.png">
</p>

(Don't think it is related to #754)

• Can you reproduce the issue?

Steps to reproduce

1. Add relative image in HTML

Expected behavior:
Render the image

Actual behavior:
Shows missing image behaviour.

Versions

• Mark Text: 0.13.65
• Operating system: macOS 10.13.3