Support merging one label into another by relabeling issues

[jwodder]

Support the following methods for merging labels:

• Add one or more fields (TBD) to the configuration file for instructing the apply command to merge one label into another

• Add a labelmaker merge <old-label> <new-label> command that does the following:

  • If <new-label> does not already exist in the repository, create it with a random color and empty description (Add options for setting these?)
  • Update all issues & PRs with <old-label> to have <new-label> instead
  • If a --no-delete option was not given, delete <old-label>

  Should this functionality be part of the make command instead?