feat: allow user not to move md file to the folder with the same name

[wongsingfo]

The default behavior of notion-export-enhandcer is to move the root md file into the folder with the same name and to rename the md file as !index.md. For example, suppose we have files:

top [a_long_ID].md
top [a_long_ID]/sub1 [a_long_ID].md
top [a_long_ID]/sub2 [a_long_ID].md

Then, the output is

top/!index.md
top/sub1.md
top/sub2.md

But this is not a desired behavor in some cases. For example, in my case, I want to export Notion to Trilium, a note taking app. The desired file structure is:

top.md
top/sub1.md
top/sub2.md

This PR aims to add an cli option --dont-move-md-to-folder to address this problem.