Add option to disable zsh global compinit

Fixes #855.

modules/programs/zsh/default.nix

@@ -70,6 +70,18 @@ in
       description = lib.mdDoc "Enable bash completion for all interactive zsh shells.";
     };
 
+    programs.zsh.enableGlobalCompInit = mkOption {
+      type = types.bool;
+      default = cfg.enableCompletion;
+      description = lib.mdDoc ''
+        Enable execution of compinit call for all interactive zsh shells.
+
+        This option can be disabled if the user wants to extend its
+        `fpath` and a custom `compinit`
+        call in the local config is required.
+      '';
+    };
+
     programs.zsh.enableFzfCompletion = mkOption {
       type = types.bool;
       default = false;
@@ -175,7 +187,7 @@ in
 
       ${cfg.promptInit}
 
-      ${optionalString cfg.enableCompletion "autoload -U compinit && compinit"}
+      ${optionalString cfg.enableGlobalCompInit "autoload -U compinit && compinit"}
       ${optionalString cfg.enableBashCompletion "autoload -U bashcompinit && bashcompinit"}
 
       ${optionalString cfg.enableSyntaxHighlighting