swirl 2.2.0

• Instructional content is no longer shipped with swirl. Instead, it is located in our course repo. When the user starts swirl, he or she is given the option to install the R Programming course automatically or be taken to the course repo page. Courses can also be installed with the install_from_swirl() function.
• Content authoring tools have also been removed from the swirl package. We've created a new package called swirlify, which is a comprehensive toolbox for swirl instructors. Instructions for authoring content are on the Instructors page of the swirl website.
• Package dependencies for a lesson are now managing by including a file called dependson.txt in the lesson directory, which lists required packages one line at a time. This strategy is mainly for backwards compatibility and will take a different form for new content in future releases. When the user begins a lesson with package dependencies, swirl attempts to load each package in turn and prompts the user to automatically install any packages not found.
• Added help files for answer tests contained in answerTests.R.
• Added progress bar feature using utils::txtProgressBar().
• Added test mode for compatibility with the swirlify package.
• Integrated with Coursera API to allow enrolled students to receive credit for swirl lessons associated their Coursera course.
• rmd2df() can finally handle figure and video units of instruction.