Implement --output={null,pipe,<FILE>}

Fixes #377.

Cargo.toml

@@ -31,6 +31,9 @@ libc = "0.2"
 [target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3", features = ["processthreadsapi", "minwindef", "winnt"] }
 
+[target.'cfg(target_os="linux")'.dependencies]
+nix = { version = "0.24.1", features = ["zerocopy"] }
+
 [dependencies.clap]
 version = "3"
 default-features = false