[feat] init.fnl

This is a <ins>beautiful</ins> plugin. I have a feature request, however: that *hotpot* might load `init.fnl`, if it exists and perhaps if neither `init.lua` nor `init.vim` are found in `stdpath("config")`. ### Reason Since neovim has a builtin package system (`:h package`), it can load plugins without an `init.lua`/`init.vim`. (One merely need `git submodule add $hotpot pack/$pkg/start/hotpot.nvim`.) Additionally, *hotpot* does not require any call to load itself, since it correctly utilizes `&rtp/plugin/` to "bootstrap" itself. Therefore, if *hotpot.nvim* is loaded via `&rtp/plugin/` but `init.lua` & `init.vim` do not exist while `init.fnl` does, perhaps *hotpot* can source `init.fnl` so there is only one entry point. I think I may be the only person, who will ever use this feature. However, this must be done this plugin's side, and I don't foresee any difficulties or additional overhead for this plugin. I am happy to attempt to implement, but I wasn't sure if this would be a welcome addition; and you are certainly more knowledgeable than I am. _^thanks!