Custom comment template

[nitrocode]

This shares a similar title to #456 but it looks like that issue was fixed using a different solution.

Use cases

• Append comment
  • We've disabled applies unless they are done using specific users and would like to point this out by appending the atlantis plan comment with more details such as a confouence link or something
• Add elapsed time to run a terraform workflow Show elapsed time to complete terraform workflow (plan/apply) #2518
• Use hcl instead of diff for code block coloration Use hcl instead of diff after triple backtick #2438
• To get an apply request on our radar, I'd also like to append the atlantis apply comment with an @ tag for a specific GitHub user or team
  • This is done using codeowners now

Thank you very much.