Add deploy start & success notifications & multiple slack channel support

[mike-sheppard]

Summary

• Add deploy start notification #3
• Update readme with info for newer versions of Trellis + variable name updates
• Add git username #4
• Add a link to the deployed site env. URL
• Add a link to the deployed commit on GitHub for quick lookup (if using GitHub, otherwise show git sha as before)
• Simplify message into a single line for quick scanning (after a couple deploys the longform messages cause quite a bit of scrolling)
• Ignore errors, so deployments do not fail #8
• Multiple Slack channels/organisations support - #9

Preview