プラグイン追加

以下の３つのプラグインを submodule として追加します。

* [redmine_custom_field_aggregator](https://github.com/occ-corp/redmine_custom_field_aggregator)
* [redmine_gantt_view_overwrite](https://github.com/occ-corp/redmine_gantt_view_overwrite)
* [redmine_text_format_selector](https://github.com/Tomohiro/redmine_text_format_selector)

以下、追加手順

```bash
$ git submodule add -f https://github.com/occ-corp/redmine_custom_field_aggregator plugins/redmine_custom_field_aggregator
$ git submodule add -f https://github.com/Tomohiro/redmine_text_format_selector plugins/redmine_text_format_selector
$ git submodule add -f https://github.com/occ-corp/redmine_gantt_view_overwrite plugins/redmine_gantt_view_overwrite
$ git submodule init
$ git submodule update
```