Feature Request: Navigate to other files in the same directory using shortcuts

[sakkas-zendesk]

Ability to navigate between files in the same directory using CMD+Arrow keys or similar functionality. Not sure what UI/UX would be like, but this would be interesting and useful to have. Thanks for the app!

PS: I can make an attempt at this and open a PR but I haven't developed any app for macOS before.

[sdkks]

I ended up making some improvements out of curiosity
https://github.com/sdkks/mdviewer

Changes are maybe a little too many and deviates from your very minimalist design. I tried to close the issue but it fails. Take care friend, thank you for creating the clean baseline for this app.