Allow configuring to reject code that is incompatible with bash 3.2.

[stevecj]

Would it be possible to have a configuration option to check for compatibility with bash 3.2 which is the latest available on MacOS/darwin?

I am developing in Linux, but many of the scripts I write are also used by other developers who work in MacOS. I frequently end up using features that turn out not to work for them. The most recent problem I had was using the ",," substitution (convert to lowercase) which bash on MacOS does not understand.