Slack now have a tool that does this https://api.slack.com/docs/messages/builder
See what the payload you send to a Slack chat.postMessage will be displayed like.
Slack Formatting documentation: https://api.slack.com/docs/formatting
Give it a go: http://davestevens.github.io/slack-message-builder
Written in ES6 (using Browserify) and Sass.
npm install
Install all required dependencies.
npm run build
Builds JS and CSS from app directory into dist directory.
npm start
Runs a local server.