Enhance find-user-init-file

Provide a layer of indirection between emacs' notion of the `user-init-file` and what the Prelude user's actual personal init-file is.  Since the recommendation is to not put personal customizations into anywhere but the `personal/` subdirectory, we should by default point them there.  And while we're at it make this new variable customizable.

I pulled the Prelude custom setup into it's own file since I wanted to utilize this new custom variable in `prelude.core.el`.