Pandoc Walkable Instances

In the Pandoc Filter Options blog entry, I mentioned that the API documentation for ToJSONFilter mistakenly listed Meta and MetaValue types as valid types for transformation functions. I submitted a pull request to fix the documentation. John MacFarlane merged the pull request as well as kindly suggested that adding the instances would be a better fix. That is what I was missing! For some reason, I did not even think of that option. Thank you very much to John for the suggestion!

This afternoon, I implemented the Walkable instances required to add support for Meta and MetaValue transformation functions. The commit includes some property tests in the style of the existing tests, and I manually did some tests using custom filters and YAML metadata in a markdown file, just because I feel more confident when I can see correct/expected results. I submitted a new pull request with these changes.