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!

alacritty/colors-dark.yml

@@ -0,0 +1,27 @@
+colors:
+  # Default colors
+  primary:
+    background: '0x1d1f21'
+    foreground: '0xc5c8c6'
+
+  # Normal colors
+  normal:
+    black:   '0x282a2e'
+    red:     '0xa54242'
+    green:   '0x8c9440'
+    yellow:  '0xde935f'
+    blue:    '0x5f819d'
+    magenta: '0x85678f'
+    cyan:    '0x5e8d87'
+    white:   '0x707880'
+
+  # Bright colors
+  bright:
+    black:   '0x373b41'
+    red:     '0xcc6666'
+    green:   '0xb5bd68'
+    yellow:  '0xf0c674'
+    blue:    '0x81a2be'
+    magenta: '0xb294bb'
+    cyan:    '0x8abeb7'
+    white:   '0xc5c8c6'