Ability to export notes

[iridakos]

Users should be able to export notes with something like

stup export --from "2020-04-15" --to "2020-05-18" --output ./report.md

and get a markdown file named report.md with contents like

# Export of notes from April 15th, 2020 to May 18th, 2020

## April 15th, 2020

### Notes related to <repository-name>

- A note
- Another note
- Another note

### Notes related to <another-repository-name>

- A note containing review
- Another note containing review
- Another note containing review

## April 16th, 2020

### Notes related to <repository-name>

- A note
- Another note
- Another note

### Notes related to <another-repository-name>

- A note containing review
- Another note containing review
- Another note containing review

...

## May 15th, 2020

### Notes related to <repository-name>

- A note
- Another note
- Another note

### Notes related to <another-repository-name>

- A note containing review
- Another note containing review
- Another note containing review
...