Add message customization

[thehanimo]

Original PR: #9

New

• Adds the ability to optionally customize the success and failure messages.
• Adds the ability to optionally add a notice upon failure (for printing out the guidelines).
• Removes "title needs formatting" label if PR has an ignore label applied
• Backwards compatible, defaults to current messages if the new config section isn't found

Fixes

• removeLabel had an unreferenced variable usage in the error handling
• adds a missing await when invoking addLabel
• no longer prints out success message when there's a failure

Refactored

• pulled failure handling out of addLabel and into a new function called titleCheckFailed