add tool to look for bashisms in shell scripts

We have a long-standing policy to use `#!/bin/bash` when a script
contains bash-specific syntax. However, we have never implemented
a unit test for it. We simply trust people to use the correct shebang.

Arch Linux wiki suggests:

    checkbashisms -f -p $(grep -rlE '^#! ?/bin/(env )?sh' /usr/bin)

checkbashisms is part of devscripts-minimal in Fedora.

Resolves https://github.com/jumanjiman/wormhole/issues/39

Related: Debian provides https://packages.debian.org/sid/devscripts
which we may want to use to test our in-house puppet and rpm repos
to assure compliance.