Add support for publishing directly to my Slack instance

Hello,

Love the app.

One crucial feature that I need for this to unlock truly limitless productivity is to be able to just run a single terminal command that both generates the `.png` and then uploads it directly to Slack.

Thank you so much for the great work here -- I already couldn't do my job without this tool!

The issue names no files, tests, or entry points. Start by locating the terminal command that generates the .png and determine how a Slack upload could be configured; done means one command performs both generation and upload.

Written by the indexing model from the issue text.