Add option to make `GpChatPaste` not open a new buffer

Hey, great plugin and it has quickly become my favourite ChatGPT plugin in Neovim. One feature that would be nice though is to make `GpChatPaste` not open a buffer. This is useful for example if you already have a chat open in a separate split/tab and you want to paste something into it. Right now it opens a new buffer with that chat instead, leaving you with two buffers of the same chat