ENH: Overwrite option in install_course_github

[muschellij2]

If you run (duplicated lines intentional):

library(swirl)
install_course_github("seankross", "AdvDataSci_Part_1")
install_course_github("seankross", "AdvDataSci_Part_1")

you get a warning about the directory not empty. It may be useful to add:

course_path = file.path(swirl_courses_dir(), course_name)
if (dir.exists(course_path) & overwrite) {
     unlink(course_path, recursive = TRUE)
}
file.rename(file.path(swirl_courses_dir(), old_name), 
            course_path)

[elinw]

This is related to the problem with unlink() that is mentioned in my pull request When unlink() fails it returns a 1 silently and the success message is given because the failure is not caught. Everywhere that unlink() is used it should be checked for an error before moving on. #522

Also this relates to
swirldev/swirlify#22

Which for some reason I put in swirlify, but it's really swirl install that has the issue of requiring uninstall to update.