conform: disable autoformat on save for specified filetypes (nvim-lua#694)

Provide a method to disable autoformat on save lsp fallback for
specified filetypes. By default disable for C/C++ as an example,
because it does not have a well standardized coding style.

Based on conform recipe:
https://github.com/stevearc/conform.nvim/blob/master/doc/recipes.md

Author: dam9000