Support custom filename formatting

[jayeheffernan]

This PR adds a way to control the format of the question filenames, by passing a function in your config. Here's a sample config that uses it:

return {
  "kawre/leetcode.nvim",
  build = ":TSUpdate html", -- if you have `nvim-treesitter` installed
  dependencies = {
    -- "nvim-telescope/telescope.nvim",
    "ibhagwan/fzf-lua",
    "nvim-lua/plenary.nvim",
    "MunifTanjim/nui.nvim",
  },
  opts = {
    lang = "rust",
    storage = {
      home = vim.fn.stdpath("data") .. "/leetcode/src/bin",
      cache = vim.fn.stdpath("cache") .. "/leetcode",
      format = function(opts)
        local fn = ("%s__%s%s.%s"):format(opts.id, opts.slug, opts.ft_alt, opts.ft)
        return fn
      end,
    },
  },
}

Motivation

I wanted to use src/bin in a Rust project as a home folder, so that LSP stuff and commands like cargo fmt and cargo test work smoothly. But I ran into this problem (e.g. running cargo test):

error: invalid character `'.'` in crate name: `9.palindrome-number`

So I want the filenames not to include ".".