not just one directory.
Also use shorter versions of the `tar` parameters, since we were using a
mixture of the short and long ones among `archive` and `unarchive`.
About not using `$` inside `(( ))`, this is what the the section
ARITHMETIC EVALUATION in zshmisc(1) says:
> Named parameters and subscripted arrays can be referenced by name
> within an arithmetic expression without using the parameter expansion
> syntax.
And according to http://www.bash2zsh.com/zsh_refcard/refcard.pdf:
> `var` (does not require `$` in front unless some substitution e.g.
> `${#var}` is needed, `$` is error if `var` is to be modified)
Closes#308
1c23ea1604