Automate switching between day/night color schemes

This sets up individual light and dark color scheme files for vim/neovim
and alacritty, adds a script (`set-color`) for placing them, and
includes two no-arg scripts for later keybinding (`day-mode` and
`night-mode`).

Thanks to this blog post for the idea to `cat` the YAML into place:
https://shuheikagawa.com/blog/2020/02/14/switching-colorschemes-of-vim-and-alacritty/

TIL YAML is concatenative!

default.conf.yaml

@@ -2,6 +2,7 @@
     - mkdir -p ~/.config
     - mkdir -p ~/.config/direnv
     - mkdir -p ~/.task
+    - ./bin/set-colors dark
 
 - clean: ['~', '~/.config']