Aliases: url_absolute url_relative
Keywords:
### ** Examples url_absolute(c(".", "..", "/", "/x"), "http://hadley.nz/a/b/c/d")
[1] "http://hadley.nz/a/b/c/" "http://hadley.nz/a/b/" [3] "http://hadley.nz/" "http://hadley.nz/x"
url_relative("http://hadley.nz/a/c", "http://hadley.nz")
[1] "/a/c"
url_relative("http://hadley.nz/a/c", "http://hadley.nz/")
[1] "a/c"
url_relative("http://hadley.nz/a/c", "http://hadley.nz/a/b")
[1] "c"
url_relative("http://hadley.nz/a/c", "http://hadley.nz/a/b/")
[1] "../c"