Option for recursive rmdir

At the moment to `rm -rf` you need to shell out, with this:
```
julia> mkpath("foo/bar/baz.jl")

julia> rmdir("foo")
ERROR: rmdir: Directory not empty
 in rmdir at file.jl:63
 in rmdir at file.jl:55

julia> rmdir("foo", true) # rm -rf
```
see also #7046.