Add glow: the markdown previewer

Currently using my own fork, will change to upstream once
the PR is accepted.

lua/core.lua

@@ -28,3 +28,4 @@ global.neoformat_basic_format_trim = 1
 
 -- leader settings
 global.mapleader = ' '
+global.glow_border = "rounded"

lua/pluginList.lua

@@ -162,4 +162,9 @@ return require('packer').startup(function(use)
 
     -- integrations
     use {"tpope/vim-fugitive", cmd = "G"}
+    use {
+        "JuanCldCmt/glow.nvim",
+        branch = 'main',
+        cmd = {"Glow", "GlowInstall"}
+    }
 end)