The repository name of the form user/repo, e.g.,
"yihui/xfun".
subpath
A character string to be appended to the URL of Github
releases (i.e., https://github.com/user/repo/releases/). For
example, you may use subpath = "latest" to get the tag of the latest
release.
pattern
A regular expression to extract the tags from the HTML source.
It must contain a group (i.e., must have a pair of parentheses).