Add subcommands to main command

The main command now supports subcommands for the various puzzles.  The
first subcommand is `sonar`, for working with the sonar data from the
Day 1 puzzles.

src/main.rs

@@ -5,8 +5,22 @@ use argh::FromArgs;
 mod sonar;
 
 #[derive(FromArgs)]
-/// Elvish Submarine Sonar Tracker
+/// Elvish Submarine Control
 struct Arguments {
+    #[argh(subcommand)]
+    command: SubCommand,
+}
+
+#[derive(FromArgs)]
+#[argh(subcommand)]
+enum SubCommand {
+    Sonar(SonarTracker),
+}
+
+#[derive(FromArgs)]
+/// sonar data reader
+#[argh(subcommand, name = "sonar")]
+struct SonarTracker {
     /// path to input data file
     #[argh(positional)]
     input: String,
@@ -18,7 +32,11 @@ struct Arguments {
 
 fn main() -> io::Result<()> {
     let args: Arguments = argh::from_env();
+    match args.command {
+        SubCommand::Sonar(args) => {
             let count_incr = sonar::count_increases(&args.input, args.window)?;
             println!("number of increases: {}", count_incr);
+        }
+    }
     Ok(())
 }