convert html to slack markdown
example usage:
var slackify = require('slackify-html');
var text = slackify('this <a href="http://github.com">link</a> is <b>important</b>');
// text variable contains 'this <http://github.com|link> is *important*'
https://app.getguru.com/card/TqGjya8c/slackifyhtml-basic-setup