While normally it's a good thing that zsh doesn't auto-expand on spaces,
in this case we want to split on spaces into distinct words, that way
the values are passed to `prompt` in multiple params.
Fixes#178
The old test, [[ ${var} ]] tests for unset AND zero-length strings.
Checking for zero length strings probably isn't that important. It is
enough to ensure that the variable is set.
Closes#18