Open, read and edit existing markdown files

[yeboahnanaosei]

This might be related to #198. I think it will be cool if Notes-up can be used as a markdown viewer for already existing markdown files.

So that in Notes-up one can just click an 'Open file' menu, then browse to an existing markdown file to view. Also any changes made will be saved to that same file.

[ghost]

As well as add existing markdown files as notes in an notebook

[rpattabi]

This will also help us keep our markdown files outside notes-up database which we can put it under version control, take backups, edit independently, etc.

[PaulB65]

+1 request to add ability to open, read and edit existing markdown files