make output markdown keep original indentation of the 1st line in a list item

[allets]

input markdown:

-   item 1
    -   item 1.1
-   item 2
    -   item 2.1

Markdown renderer output (v1.2.1):

- item 1
  - item 1.1
- item 2
  - item 2.1

I expect the output of Markdown renderer to be the same as the input.

My use case:
I developed a tool to replace image links with local paths after downloading these images.
I plan to use a parser.
If it keeps the original format as much as possible, then users can easily check the necessary differences.
And I found the great project, mistletoe!
Thanks for your contributions to the project.