Image replacement

[xamgore]

The issue is mostly about the API, as the implementation is quite simple. Either replace the image in the zip archive itself, or replace a placeholder with some XML which encodes the image and its formatting.

Right now it looks like this:

fn main() -> Result<(), Box<dyn std::error::Error>> {
  let path = "./examples/image-replacement/input.docx";
  let mut file = DocxFile::from_path(path)?;

  let path = "./examples/image-replacement/output.docx";
  let mut result = BufWriter::new(File::create(path).unwrap());

  let cat_img = include_bytes!("cat.jpg");
  file.replace_file(&mut result, "word/media/image1.jpg", cat_img)?;
                         ^^^^^^

  Ok(())
}

If I want to replace >1 images, there will be a problem.