Use shebangs to inform which linters are applicable

[mcandre]

E.g., if fizzbuzz has a shebang like #!...zsh, then checkbashisms should skip that file.

Related to #3.

[duggan]

checkbashims (as far as I know) specifically only tries to check files which declare themselves as being #!/bin/sh or equivalent.

I'm not familiar with zsh, is there an idiosyncratic syntax which causes false positives with checkbashims? Hopefully an update to checkbashims will fix any lingering issues around this, if not, I suspect it is best resolved upstream.