Add Vimtex for Latex support (#512)

* Add vimtex plugin

Vimtex provides a full integration for latex files, see: https://github.com/lervag/vimtex.
Zathura is used as default pdf viewer as it allows live reloading.

* Provide the <leader>L shortcut for Latex operations.

* Move vimtex into the extra plugins section

Add a short comment above extra plugins to introduce their functionality

* Only define whcihKey mapping for Vimtex if O.extras is true

Co-authored-by: Christian Chiarulli <chris.machine@pm.me>

lua/plugins.lua

@@ -89,8 +89,10 @@ return require("packer").startup(function(use)
     -- Zen Mode
     use {"Pocco81/TrueZen.nvim", opt = true}
 
-		-- Sane gx for netrw_gx bug
-    -- use {"felipec/vim-sanegx"}
+
+	-- Sane gx for netrw_gx bug
+    use {"felipec/vim-sanegx"}
+
 
     require_plugin("nvim-lspconfig")
     require_plugin("lspsaga.nvim")
@@ -125,28 +127,42 @@ return require("packer").startup(function(use)
 
     -- Extras
     if O.extras then
+        -- Interactive scratchpad
         use {'metakirby5/codi.vim', opt = true}
         require_plugin('codi.vim')
+        -- Markdown preview
         use {'iamcco/markdown-preview.nvim', run = 'cd app && npm install', opt = true}
         require_plugin('markdown-preview.nvim')
+        -- Floating terminal
         use {'numToStr/FTerm.nvim', opt = true}
         require_plugin('FTerm.nvim')
+        -- Enhanced increment/decrement
         use {'monaqa/dial.nvim', opt = true}
         require_plugin('dial.nvim')
+        -- Peek lines
         use {'nacro90/numb.nvim', opt = true}
         require_plugin('numb.nvim')
+        -- HTML preview
         use {'turbio/bracey.vim', run = 'npm install --prefix server', opt = true}
         require_plugin('bracey.vim')
+        -- Better motions
         use {'phaazon/hop.nvim', opt = true}
         require_plugin('hop.nvim')
+        -- Colorizer
         use {'norcalli/nvim-colorizer.lua', opt = true}
         require_plugin('nvim-colorizer.lua')
+        -- Search & Replace
         use {'windwp/nvim-spectre', opt = true}
         require_plugin('nvim-spectre')
         use {'simrat39/symbols-outline.nvim', opt = true}
         require_plugin('symbols-outline.nvim')
+        -- Treesitter playground
         use {'nvim-treesitter/playground', opt = true}
         require_plugin('playground')
+        -- Latex
+        use {"lervag/vimtex", opt = true}
+        require_plugin("vimtex")
+
         -- folke/todo-comments.nvim
         -- gennaro-tedesco/nvim-jqx
         -- TimUntersberger/neogit