Full content authoring app

I created a full web app, intended to be served locally via shiny, in hopes of making content authoring a more straightforward proposition. There are still aspects that could be improved, but I think it's a whole lot better than the system we currently have in place (freehand R Markdown with essentially no documentation). 

I've left the old `author_lesson()` functionality intact and I'll maintain what documentation exists on a separate page of swirlstats.com. If you want to test out the new app...

```
library(swirlify)
swirlify("Test Lesson", "Test Course")
```

I'm open to feedback, but I'd like to merge something before Sunday.