Prompt for caption and create filename from it

[Grueslayer]

Related issue

Can be a possible solution for #76

Summary of changes

New option prompt_for_caption shows a prompt, creates a filename from it and replaces $CAPTION in template. If prompt_for_file_name is also given, the suggested filename is already available in prompt.

This works fine for me:

      require("img-clip").setup({
        default = {
          relative_to_current_file = true,
          prompt_for_caption = true,
          prompt_for_file_name = true,
          show_dir_path_in_prompt = true,
        },
        filetypes = {
          markdown = {
            url_encode_path = true,
            template = "![$CAPTION]($FILE_PATH)$CURSOR",
            download_images = false,
          },
        },
      })

[HakonHarnes]

Thanks for the PR. I like the idea of the caption.

I think the caption and file name are too tightly coupled at the moment. The idea of computing the file name directly from the caption in a non-configurable (and opinionated) way is not flexible enough. This should be configurable by the user.

I suggest you make the following changes:

• Keep the $CAPTION and prompt_for_caption options.
• Add a caption configuration option. This way, users can auto generate captions (through lua functions) if they wish, similar to the file_name option.
• When generating the file name, pass the caption as an argument to the lua function. This is done by passing the caption as an argument in the get_opt function, i.e. config.get_opt("file_name", { caption = caption }). This way, users can generate the file name from the caption through the configuration.
• The get_file_path_and_caption should be split into two functions: get_file_path (original implementation) and get_caption. They should be entirely decoupled to decrease complexity for the sake of maintainability.

This approach should allow more or less full flexibility to the user.

Let me know if you have any questions or if there's anything else I can assist with. I'm definitely open to merging this PR if you make the above changes!