stank: Flags to restrict results to bash subsets

[mcandre]

bash subset scripts should be relatively suitable for linting with bashate.

We may want to distinguish between:

• non-bash scripts (e.g. POSIX sh, ksh, csh, fish, etc.)
• full bash scripts (bash, bash3, bash4, etc.)
• bash proper subsets (ash, dash, posh, etc.)
• bash-like (zsh)