* Add YAML to formats supported by load_data()
A fairly trivial addition; JSON and YAML are handled so similarly
that this was a matter of copying the JSON-relevant handlers and
editing the copies to handle YAML as well. The test file was
literally generated with 'json2yaml'.
The documentation has been updated to indicate that load_data() now
handles YAML code.
The CHANGELOG has been updated as well.
* After checking, I found that it's generally agreed the mime type is still application/x-yaml.
* Update comment, unify library importing.
I noticed one more place where the list of formats was supported,
and added YAML to that list.
I noticed that there's a singular place to load the `libs::` crate,
and unified by importing of serde_yaml in that place.