Add possibility to download documents (save)

[MyXoToD]

Download document as *.md

This could possibly combined with an HTML preview:
Preview document -> download this as .md or .html

[MyXoToD]

Downloading documents is now possible (md & html). Every document is now created as a file in the document tree. This requires using maxdown running locally or on a server where it has permission to create files.

This works for now as we all use it locally but we need to improve this in the future.