Show progress bar

Currently the Cargo subprocess spawned by cargo-expand does not display Cargo's usual progress bar. I believe this is because we pipe the child's stderr to perform filtering here:

https://github.com/dtolnay/cargo-expand/blob/3ae4ca126a4bd557ef7f7954911d65db56525b37/src/main.rs#L358

and that means Cargo does not get a winsize from this ioctl:

https://github.com/rust-lang/cargo/blob/6be12653dcefb46ee7b605f063ee75b5e6cba513/src/cargo/core/shell.rs#L377

I tried a naive fix in #61 but it didn't work, the second ioctl fails with:

Inappropriate ioctl for device (os error 25)

Will need to figure out what's the correct way to provide a winsize to the child process.