Refactor command-line handling into modules per subcommands

cli/src/bin/task.rs

@@ -0,0 +1,43 @@
+use clap::{Error as ClapError, ErrorKind};
+use std::process::exit;
+use taskchampion_cli::parse_command_line;
+
+enum Output {
+    Stdout,
+    Stderr,
+}
+use Output::*;
+
+fn bail<E: std::fmt::Display>(err: E, output: Output, code: i32) -> ! {
+    match output {
+        Stdout => println!("{}", err),
+        Stderr => eprintln!("{}", err),
+    }
+    exit(code)
+}
+
+fn main() {
+    let command = match parse_command_line(std::env::args_os()) {
+        Ok(command) => command,
+        Err(err) => {
+            match err.downcast::<ClapError>() {
+                Ok(err) => {
+                    if err.kind == ErrorKind::HelpDisplayed
+                        || err.kind == ErrorKind::VersionDisplayed
+                    {
+                        // --help and --version go to stdout and succeed
+                        bail(err, Stdout, 0)
+                    } else {
+                        // other clap errors exit with failure
+                        bail(err, Stderr, 1)
+                    }
+                }
+                Err(err) => bail(err, Stderr, 1),
+            }
+        }
+    };
+
+    if let Err(err) = command.run() {
+        bail(err, Stderr, 1)
+    }
+}