Accommodating custom GitHub URL's

`devtools::install_github()` might accept an extra parameter, `repo_path`, that defaults to `https://github.com/` but can point to a custom Enterprise GitHub URL. 

This default ensures that this feature will change nothing in the way this function is used now, but the new parameter would allow `devtools` to be used as a way to disseminate packages between R users who are members of closed Enterprise GitHub communities.