feat: Add open_cmd to allow split open

[echaya]

1. refactored private _open_file that support open_cmd
2. refactored _create_buffer_keymaps

[echaya]

Hi, I have been using vimwiki for min 5 years and today i realized it has stopped updating so I pulled the trigger to migrate to kiwi.nvim. First of all, thanks a lot for creating this solid repo!

This PR was meant to add and which I use a lot when working with vimwiki. While doing so, i also refactored a bit here and there to make it DRY.

[serenevoid]

Nice work! Glad to see your project. Sorry for my delay in responses though. As I mentioned in some other PR, I couldn't follow up with the pending pull requests and bug fixes. I'll try to resolve them slowly

[echaya]

thanks @serenevoid - no rush at all.

completely off the topic, i really love the name of this plugin :)