Add Pass percentage to Slack results

[Alquh]

The pass % would be nice at a glance to see if we are getting better or worse with our tests

[Alquh]

@jackcoded this is how I have my script setup but it's not outputting my Environment to my slack message. Is it formatted correctly? -r slackmsg --reporter-slackmsg-webhookurl 'https://hooks.slack.com/blah' --reporter-slackmsg-environment 'DEV'