feat: add clap_complete for generating completion files

`ahab generate <shell>` can now be used to generte completion files for
this tool.

src/cli/ahab.rs

@@ -1,5 +1,6 @@
 use super::{Django, Docker, DockerCompose, Postgres};
 use clap::{Parser, Subcommand};
+use clap_complete::Shell;
 
 /// A program for interacting with various dockerized applications.
 #[derive(Parser, Debug)]
@@ -11,6 +12,12 @@ pub struct Ahab {
 
 #[derive(Debug, Subcommand)]
 pub enum Commands {
+    /// Generate completion files
+    Completion {
+        #[arg(value_enum)]
+        shell: Shell,
+    },
+
     /// Docker related subcommands
     D {
         #[command(subcommand)]