Give the option of not overriding elpa directory (package-user-dir)

[dmgerman]

Is your feature request related to a problem? Please describe.

I want to have different elpa directories for different versions of emacs. Currently prelude overrides package-user-dir

Describe the solution you'd like

A variable that controls this behaviour (defvar) so the user can override it. eg:

 (when prelude-override-package-user-dir (setq package-user-dir ...

Describe alternatives you've considered

My current solution is has been to comment the (setq package-user-dir ...) line

Additional context

It helps to have different directories for elpa downloaded files for different versions of emacs.