feat(extension): Extension for the Mundo plugin (#740)

* Add an extension for the mundo plugin

* List the mundo extension

README.md

@@ -756,6 +756,7 @@ extensions = {'quickfix'}
 - fugitive
 - fzf
 - man
+- mundo
 - neo-tree
 - nerdtree
 - nvim-dap-ui

lua/lualine/extensions/mundo.lua

@@ -0,0 +1,20 @@
+local M = {}
+
+M.sections = {
+  lualine_a = {
+    function()
+      local ft = vim.opt_local.filetype:get()
+      return (ft == "Mundo")     and "Change tree"
+          or (ft == "MundoDiff") and "Change diff"
+    end
+  },
+  lualine_y = { "progress" },
+  lualine_z = { "location" },
+}
+
+M.filetypes = {
+  "Mundo",
+  "MundoDiff",
+}
+
+return M