Change structure to support not just display images

This PR changes the way to analyze an URL in a erc buffer. Instead of call a function to show images and use `erc-image-regex-alist` to retrieve the proper image URL, we use `erc-image-regex-alist` in `erc-image-show-url` to call a function that could display and image or just show, for example, the info about a twitt URL.

This is a small change in erc-image that could be a seed to change the future of erc-image and make it to become a package to process any URL in a erc buffer.