YAML source support

[robjtede]

Supporting YAML source was requested by a prospective user.

With serde-yaml's deprecation, we'll need to investigate solutions.

[tenuous-guidance]

We can probably start with serde-yaml and swap to something else later, when there's a clear successor. It would be a breaking change but we've had plenty of those so I'm not too worried and I expect the breakage would be pretty light.