Make element optional for workspace close and reset

closing workspaces is going to need special treatment when multiple
projects open the same workspace.

Also, it is currently a nuisance when you reset the workspace you're
currently in, because that deletes the directory you're in, requiring
you to `cd $CWD`. I do not know a good solution to this, given workspace
commands can be called from anywhere inside the workspace, so it's not
simply a matter of not deleting the workspace dir.