nvim: Add criticmarkup highlighting

Added simple plugin for some highlighting of criticmarkup.
Criticmarkup is a neat way of doing 'track-changes' in markdown. It's a
bit cumbersome to read just the way it is when there's a lot of changes
however and this plugin helps immensely with that.

There is also a github gist out there which helps using pandoc to turn
Microsoft Word tracked changes into criticmarkup formatted markdown.
Amazing to work with through vim instead of the annoying Office suites!

nvim/.config/nvim/lua/plugins.lua

@@ -98,6 +98,7 @@ require("packer").startup(function()
     -- writing
     use {'vim-pandoc/vim-pandoc-syntax', ft = 'pandoc'}
     use {'vim-pandoc/vim-pandoc', ft = 'pandoc'}
+    use {'vim-pandoc/vim-criticmarkup', ft = 'pandoc'} -- highlights for criticmarkup
     use 'micarmst/vim-spellsync' -- personal dict improvements for git sync
     use { -- provide distraction free writing
         'Pocco81/TrueZen.nvim',