Add quick-and-dirty Debian package spec for cargo-deb

That's not a package you can show to your mother, but it makes easier
to install/update/uninstall exa in systems without cargo or rustc.

Cargo.toml

@@ -54,3 +54,17 @@ default-features = false
 
 [dependencies.zoneinfo_compiled]
 git = "https://github.com/rust-datetime/zoneinfo-compiled.git"
+
+[package.metadata.deb]
+license-file = [ "LICENCE" ]
+depends = "$auto"
+extended-description = """
+exa is a replacement for ls written in Rust.
+"""
+section = "utils"
+priority = "optional"
+assets = [
+    [ "target/release/exa", "/usr/bin/exa", "0755" ],
+    [ "contrib/man/exa.1", "/usr/share/man/man1/exa.1", "0644" ],
+    [ "contrib/completions.bash", "/etc/bash_completion.d/exa", "0644" ],
+]