Add support for the "nathanaelkane/vim-indent-guides" plugin (#226)

The even and odd highlighting blocks using `nord1` and `nord2` (`nord3`
in terminal mode) to provide a subtle and non-disturbing style.

Note that the custom theme colors [1] are only applied when the
`indent_guides_auto_colors` variable has been set to `0`:

```vim
let g:indent_guides_auto_colors = 0
```

[1]: https://github.com/nathanaelkane/vim-indent-guides#setting-custom-indent-colors

Resolves GH-186

colors/nord.vim

@@ -643,6 +643,11 @@ hi! link ClapProviderAbout ClapDisplay
 hi! link ClapProviderColon Type
 hi! link ClapProviderId Type
 
+" vim-indent-guides
+" > nathanaelkane/vim-indent-guides
+call s:hi("IndentGuidesEven", "", s:nord1_gui, "", s:nord1_term, "", "")
+call s:hi("IndentGuidesOdd", "", s:nord2_gui, "", s:nord3_term, "", "")
+
 " vim-plug
 " > junegunn/vim-plug
 call s:hi("plugDeleted", s:nord11_gui, "", "", s:nord11_term, "", "")