Compile on stable

[dead10ck]

I've noticed a couple of issues about failing to compile already, but I feel it should be possible to compile on stable Rust. I'm not sure what the io feature is (it doesn't seem to be listed in the reference or anywhere my google-fu can reach), so I'm not sure how heavily this project depends upon it, but if there's a way to get around it, it would be great to have binaries built with a stable compiler.

[lotabout]

@dead10ck skim use the io feature to fetch the user input. More specifically, use the io::chars method.

I understand it is annoying that a nightly version is needed to build skim. I'll try to find an alternative solution.

[dead10ck]

Wow, it's very surprising that iterating over characters isn't supported in stable! That seems like a glaring omission. Thank you for your effort to work around it!