Pull Prelude's custom variables into a new file

This is so we can define a custom variable to use in prelude-core
Add new prelude custom variable for user-init-file

This way the prelude-find-user-init-file can actually take you to a
useful file (instead of to init.el)
Fix checkdoc problems with new custom file
Add comment documenting load dependency
Add to documentation for new variable

Describe a simple and easy way to specify which file should be used as the "init-file."
Rewrite the doc string to be less bad
Update prelude-find-user-init-file docstring

To reference the variable that it actually pulls from now
Use expand-file-name instead of concat
Find user-init-file when given prefix arg
Make prefix argument optional

It seems to be the general style of how prefix args are done...
Merge pull request #534 from RadicalZephyr/user-init-file

Enhance find-user-init-file