feat: add clap_complete for generating completion files

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

src/main.rs

@@ -1,7 +1,7 @@
 use ahab::{cli, scripts};
 
 use anyhow::Result;
-use clap::Parser;
+use clap::{CommandFactory, Parser};
 
 fn main() -> Result<()> {
     // always load dotenv on start
@@ -13,6 +13,15 @@ fn main() -> Result<()> {
     let args = cli::Ahab::parse();
 
     match args.command {
+        cli::Commands::Completion { shell } => {
+            clap_complete::generate(
+                shell,
+                &mut cli::Ahab::command(),
+                "ahab",
+                &mut std::io::stdout(),
+            );
+            Ok(())
+        }
         cli::Commands::D { command } => match command {
             cli::Docker::StopAll => scripts::docker::stop_all(),
         },