[RFC] 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.

This PR renames these variables to have a cleaner separation between neovim and vim. I know that these changes are very selective though.
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