Support for slack.

[nerocrux]

Hi,
Thank you for creating this tool. It makes my life much easier.
I would like to suggest support for slack. The workflow would be like:

1. Take a screenshot.
2. Prompt a modal with a list of slack channels fetched with slack token.
3. User would choose a channel, maybe write some comments.
4. Katana will upload the screenshot to the channel.

Reasons I suggest this feature.

1. Slack is popular and can host image files.
2. Enterprise users and some of the OSS projects may not want to upload their screenshots to a public file storage service.

If you are interested, maybe I will send you a PR when I have time.
Thanks again.