Add Markdown parsing

[gpoussel]

Libraries:

• https://github.com/syntax-tree/mdast
• https://github.com/remarkjs/remark
• https://github.com/vfile/vfile

Remark plugins I should look into:

• remark-directive or remark-flexible-containers or remark-hint for callouts / admonitions
• remark-capitalize or remark-capitalize-headings to properly capitalize titles
• remark-extract-frontmatter or remark-frontmatter to parse YAML frontmatter and remove it from AST
• remark-gfm for reading tables (but anything else is not useful for markpoint)
• remark-headings to parse headings (and detect the document title for example)

Useful references:

• remark-docx, since docx is very similar to pptx

[gpoussel]

Markdown conversion has to be manually updated by reading through the AST. It limits the list of usable plugins.