vim: initialize colorizer only if termguicolors is set

This additional check is needed because when nvim is started as `nvim -u
NORC`, my configuration that sets `termguicolors` is not called but
colorizer IS installed and requires `termguicolors` to be set. If it is
not set, the setup() function prints an error.

config/nvim/after/plugin/colorizer.lua

@@ -1,5 +1,5 @@
 local ok, colorizer = pcall(require, 'colorizer')
-if ok and colorizer then
+if ok and colorizer and vim.opt.termguicolors:get() then
   colorizer.setup({'*'}, { mode = 'foreground' })
 end