Add markdown parsing capability to datura

[karindalziel]

I see this working in two ways: the ability to parse markdown in individual cells of a csv, and, eventually, the ability to parse markdown files themselves (preferably with or without header data)

Would probably use the kramdown gem (https://github.com/gettalong/kramdown), which is used in several wrappers like https://github.com/vmg/redcarpet