Added check to see if custom-file was set in .emacs-live.el before
loading ~/.emacs.d/etc/custom/custom-configuration.el
Added function for setting `custom-file`

It seemed more in the style of emacs-live to have a function for setting
`custom-file` instead of using `setq` directly.
Merge pull request #98 from rothmichaels/custom-config

Added check to see if custom-file was set in .emacs-live.el