Allow the option of not overriding package-user-dir

[dmgerman]

Currently prelude overrides user-package-dir to be /elpa

This patch does not change that behaviour, but gives the user to disable it.

Adds a variable prelude-override-package-user-dir that by default is t.

The path is only set when prelude-override-package-user-dir is true

Addresses #1406

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[bbatsov]

In hindsight - probably Prelude shouldn't set such a variable at all. I'm struggling to remember why I even added this - I assume it was some desire to keep packages installed when using Prelude somewhat isolated.

[dmgerman]

I suspect that was the original reason.

The problem I have is that I that I needed to load some packages before prelude that were stored in a different place, but the biggest one was I could not keep packages for 2 versions of emacs installed in separate places.