Panics when trying to compress folder with non-archive format #96
Description
Instead of panicking like this:
# src/ is a folder
$ cargo run -q -- c src src.xz
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/commands.rs:70:85
note: run with `RUST_BACKTRACE=1` environment variable to display a backtraceShould give a helpful error message, saying that its impossible to compress a folder with .xz.
Same applies for .gz, .bz and .zst.