stack.sh with `set -o nounset` and `set -o errexit`

[zbeekman]

Hi,

Your stack in Bash is pretty nifty! It would be great if you could update it to have stronger error checking (while maintaining Bash 3 support) so that it can be sourced/used in scripts that set -o errexit, set -o nounset and set -o pipefail.

Thanks!

[bmc]

Not sure when I'll get to that. I don't use this thing much any more (and I've switched to Zsh for daily shelling). Happy to take PRs if you get tired of waiting.