Release milestones

[ajanon]

To help plan features and work on long-term goals for the project, we could use GitHub milestones.

This would be particularly helpful for deprecation or for big features that could then be split into multiple smaller issues to distribute work more easily.

For now, we could create a 1.3.0 milestone and add some features we would like to implement. Fixes could just be added as needed and as implemented.

This would help in bringing features to stable releases in a timely manner (see #194).