swirlify 0.5

[seankross]

• [ ] New function to pull course from GitHub and start working on it immediately.
• [ ] Better course testing
• [x] Change testit() to demo_lesson() in swirl
• [x] Solve RStudio issue with file.edit()
• [x] Warn that testit() will be deprecated in swirlify 0.6
• [x] Fix testit() with regard to reading the MANIFEST
• [x] Fix add_to_manifest() so that a space does not appear after the lesson name
• [x] Add Shiny authoring app
• [ ] Add spell checking a la https://github.com/hadley/devtools/pull/1119
• [x] Make swirlify() just work with regard to switching from one lesson to another in the same course.
• [x] Write a test for google_form_decode()