Adds multi file support + sending output to corresponding text files

[danielhintz]

Current usage doesn't change but now you can do multiple images and even bash expansion e.g. imgclip *jpg -w and that will pull the text from the *jpg files to corresponding *txt files

[Arthelon]

Thanks for the PR. I think it's awkward how only the text of one image is copied to the clipboard but it also doesn't make sense to concatenate the text content of all the images.

I think one way to handle this is to only enable multiple images when the write flag is present.