[RFC] Read $NVIMINIT before $VIMINIT (was: Rename $VIMINIT, $VIMRUNTIME and $MYVIMRC)

I found [this](http://tlvince.com/vim-respect-xdg) article which explains how to make vim respect the XDG Base Directory Specification and did the same changes to my setup (until #78 is figured out). Now the problem is that I want to use neovim and vim in parallel, but have different configurations (`$XDG_CONFIG_HOME/nvim` vs `$XDG_CONFIG_HOME/vim`) for them. The problem is that both read the same environment variables.

The basic idea is to change the location of `.vimrc` via $VIMINIT which is read before the configuration file is loaded.

> _(This PR renames these variables to have a cleaner separation between neovim and vim. I know that these changes are very selective though.)_

**EDIT:** @justinmk [suggested (comment)](https://github.com/neovim/neovim/pull/1153#issuecomment-55039622) as less crappy way to fix this problem.