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-light.yml

@@ -0,0 +1,27 @@
+colors:
+  # Default colors
+  primary:
+    background: '0xf3f3f3'
+    foreground: '0x303030'
+
+  # Normal colors
+  normal:
+    black:   '0xdadada'
+    red:     '0xf32840'
+    green:   '0x059b00'
+    yellow:  '0xf06d14'
+    blue:    '0x337ada'
+    magenta: '0xac37f5'
+    cyan:    '0x0ababe'
+    white:   '0x707070'
+
+  # Bright colors
+  bright:
+    black:   '0xb3b3b3'
+    red:     '0xad1d2f'
+    green:   '0x345a09'
+    yellow:  '0xb44900'
+    blue:    '0x024299'
+    magenta: '0x63039f'
+    cyan:    '0x008a86'
+    white:   '0x909090'