Preprocess user messages

[helins]

A custom hook on every user message before sending the chat for completion would make this plugin extremely extensible. For instance, a hook might rewrite messages so that a substring like [[file x]] would embed the actual file x. That would open a world of possibilities.

Of course, right know anyone could use some Neovim magic to actually embed the data directly in the chat using commands and such. However, that would often bloat the buffer (e.g. sending a large file) and generally speaking be a limiting factor.