Parser to Convert Markdown to CTV file.

This is a proof-of-concept of a parser to convert a markdown file to a ctv file, that is required for task views. The basic idea is to use a combination of YAML and markdown sections in the md file, parse it and convert it into a payload, and render it using a template as specified by the ctv package.

Structure of Markdown File

---
name: Working with data on the web
maintainer: Scott Chamberlain, Karthik Ram, Christopher Gandrud
email: scott at ropensci.org
version:  2013-09-17
---

<!- information in markdown ->

---

<!- links ->

• Ultimately, it needs to pass check_ctv_packages('test.ctv'), but I am running into some errors connected to conversion of markdown to HTML.
• Packages are assumed to be marked up as “[pkgname][pkgname]”. But, I realize that this is just a markup making use of Pandoc’s markdown. So we need an explicit markup for packages so that the add_pkg_markup function would identify packages correctly.