Transition to read-only

[bndw]

I'd like to transition this repository to read-only, but before doing so I'd like to wrap up any inflight work.

The two items off the top of my head would be:

• Wrapping up the tests on backends: Use defaults even if config option was provided but empty #191 to merge. @leonklingele is this something you'd like to contribute?
• Publish a final release

EDIT: Looks like November 8th will mark the 4 year anniversary of this project 🎉 . Cutting a final release and moving to read-only on that date would be pretty cool.