mdm milestone command

[jverkoey]

Use cases:

# Create a v1.0.0 milestone
mdm milestone create v1.0.0

This command should look up milestone templates stored on disk somewhere. E.g.:

milestones/
  v1.0.0/
    001 Write the README.md
    002 Cut the v1.0.0 release
    003 Publish the v1.0.0 release
    004 Generate and publish documentation

Each file in the directory should have a yaml preamble to specify GitHub labels to be applied to the issue. The rest of the file should be the body of the issue.