fix multilayer of section_group and img element parse error

[chinghua0731]

Hi Danmou,

I found two issues when i used this tool on my notebook.

• Issue-1, image element parse error In my case, I found that if the image element contains ../>.. string in alt attribute, there will be a error during the download_image function. Because the parsed image element string is incomplete, it didn't contain image_url attribute and lead to the error.

• Issue-2, multi-layer of section_group in notebook There is a multi-layer structure in my notebook, for example, <notebook>/<section_group>/<section_group>/<section>/<pages>. But origin feature of this tool could only parse the first layer of section_group. So I made a little modification to meet my requirement.

Anthony

[puffitos]

Just wanted to say thanks for the commit @chinghua0731, you saved me more than some time trying to solve this myself :heart: