Sort lecture files and assignments into folders about rcourse

[lwjohnst86]

Fortunately or unfortunately, generating websites in R Markdown does not allow using folders/subfolders, so this is the way it's gotta be. There's probably work arounds, but, really, that is a very low priority issue (at least to me). Even for creating books using the bookdown R package (this is how the R for Data Science book was created), you can see on the r4ds GitHub repo all the book chapters are in a single directory.

[lwjohnst86]

This is why I added lec## to each lecture file.

[joelostblom]

Agree, let's close this for now then. I will reopen if we end up with tonnes of files and a big mess, but for now I'm excited and grateful that the site looks so stunningly stylish that I will refrain from jabbing at R =)