Support for xoxs tokens (or something other than legacy tokens)

First off, thanks so much for building this awesome app!

Not all workspaces allow legacy tokens (xoxp). Would it be possible to add support for [newer API tokens](https://api.slack.com/docs/token-types) (xoxs) or the oauth flow? xoxs tokens are relatively easy to grab, just load up Slack in a browser, log in, open up the dev console and search for "xoxs" (you may need to reload to see it). It should show up in a boot config section for the browser app.