R lesson update

This is a major rewrite/restructuring of the R lesson. It's based on the lesson I taught for Data Carpentry at UF on Sept 29-30.

There are many changes compared to the initial content of the repository. The main ones are:
- I tried to make sure that the concepts were introduced sequentially and attempted to use consistent vocabulary throughout the lesson
- I added a section on how to ask and get help (not used in teaching but as a resource for after a workshop)
- I added several topics (how to add/remove columns/rows)
- I added several Challenges throughout the lessons
- I split the lesson in many shorter topics (as preferred with the new lesson template at SWC)
- I started to use the formatting for the new lesson templates

In the next few days, I will continue to tweak a few things (e.g., better motivation, demonstration of what will be learned during the less) but it would be great to start getting feedback.