Allow custom local env folder name

[avkoval]

Thank you Mark for your package, I am using it for many years!

But there is one thing I constantly have to do: change the original code to recognize the local python environment in a folder named env.

I would like to suggest having a defcustom variable for this purpose and so providing my code. I am not Elisp programmer, so please tell me if I need to fix it, but so far so good, it works here.

WBR, Oleksii