Configuration option to disable for filetypes

[bogzbonny]

closes #118

Pretty straight forward functionality. Will gladly make any requested changes if there is a desire to merge this!

I tested it manually, both setups in the readme work

---

from the README updates:

6. Disable code completion for certain filetypes (e.g., markdown)

    ```vim
    let g:llama_config = { 'disabled_filetypes': ['markdown', 'text'] }
    ```

    Or with lazy.nvim:

    ```lua
    {
        'ggml-org/llama.vim',
        init = function()
            vim.g.llama_config = {
                disabled_filetypes = { 'markdown', 'text'},
            }
        end,
    }
    ```

    This will prevent the plugin from enabling when the filetype matches any
    of the entries in the `disabled_filetypes` list. 

    Additionally, using `enabled_filetypes` one can disable code completion for
    all filetypes, then further only enabled it for specific filetypes. eg:

    ```vim
    let g:llama_config = { 'disabled_filetypes': ['*'], 'enabled_filetypes': ['rust']  }
    ```