add an example on positional vs keyword arguments

[James Lamb]

This issue documents a few things that would make the material from Week 1 of the R Programming course more approachable for students, based on observations from how the first lecture went.

1. Add an example in the code supplement showing different calls to a function and how the result differ based on how you specify arguments (to teach positional vs. keyword, default vs. non-default)
2. Remove any uses of %% from the course (#89)
3. Do objects (e.g. vectors, lists, dataframes) before control flow, since the examples use vectors and subsetting (#89)