Adding two lessons to R_Programming

This pull request adds two new lessons to R_Programming: Workspace_and_Files and Base_Graphics. I plan on creating more lessons like this. Both new lessons have been tested.  If there are guidelines I should be following with these lessons, please let me know. Advice on length and difficulty is also welcome.

If there are any recommendations for how to work with the repository, please let me know. As you can see, right now I am just working on master. That works fine, although it means that, if I add another lesson tomorrow, my next pull request (if this one has not already been processed) will include these lessons as well. (Corrections welcome.) If this is not desirable, please let me know. If it does no harm, then I am happy enough t work this way.