Support input from stdin

[uncenter]

It would be nice to be able to use this with the pipe (|) operator, e.g. echo "Hello, world!" | uwuifyy. In the past I've used https://github.com/thepacketgeek/clap-stdin for this sort of thing (https://github.com/uncenter/kittysay/blob/6955b362fca7bc5197a86624febad7c0f7ffd9d7/src/main.rs#L54) though since you don't use the derive method for clap I'm not exactly sure how to do it. Giving the derive method a try might be worth it, I've found it to be simpler and more readable :)