Allow for disabling auto-formatting

Allow for disabling auto-formatting with a global or buffer-local
variable, based on
62eba813b7/doc/recipes.md (autoformat-with-extra-features).

In the future, it would be interesting to do this based on the presence
of a file in the repository, such as `.do-not-auto-format`.

plugin/conform.lua

@@ -12,8 +12,15 @@ conform.setup {
     yaml = { "yamlfmt" },
   },
 
-  format_on_save = {
-    lsp_fallback = false,
-    quiet = true,
-  },
+  format_on_save = function(bufnr)
+    -- Disable with a global or buffer-local variable.
+    if vim.g.disable_autoformat or vim.b[bufnr].disable_autoformat then
+      return
+    end
+
+    return {
+      lsp_fallback = false,
+      quiet = true,
+    }
+  end,
 }