Added STRONGBOX_PATH for configuring config file name

I'm quite pedantic when it comes to file organisation so I'd prefer to configure the file name of the config file itself. Any pointers or improvements are welcome!

I've added a STRONGBOX_PATH which is derived from the STRONGBOX_HOME, defaulting to the defaults currently in place to avoid breaking anything.

Hey @marziply , thanks for the PR.

I don't mind the change in principal, but I don't want to keep adding code and logic that is trivial and secondary to the application.

I also want to reserve the ability to have more config or temp files in the future, without having to introduce more flags for each specific file.

Having a single directory (that is customisable) is a good middle ground for simplicity / flexibility.