Use XDG_CONFIG_HOME

This assumes that XDG_CONFIG_HOME is ~/.config

Ideally it should not. For instance, line 2 of init.vim should be:
```vim
execute 'source ' . stdpath('config') . '/plugins.vim'
```

I should be able to clone the repo and try it out:

```
git clone jarvis
XDG_CONFIG_HOME=$PWD/jarvis/config nvim jarvis
```