I might be wrong on this, but I think this line is unnecessary. Suppose my Jekyll project is in `/Users/yihangho/blog`, and I set `emoji-images-path` to `asset/emoji`. `images_dir` will then be set to `/Users/yihangho/blog/asset/emoji`. As a result, the HTML image source will look something like `/Users/yihangho/blog/asset/emoji/custom.png`, no?