Switch to a command-line API closer to TaskWarrior

* Use a parser (rather than clap) to process the command line
* Outline some generic support for filtering, reporting, modifying, etc.
* Break argument parsing strictly from invocation, to allow independent testing

cli/src/bin/task.rs

@@ -1,44 +1,8 @@
-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() {
-    env_logger::init();
-    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)
+pub fn main() {
+    if let Err(err) = taskchampion_cli::main() {
+        eprintln!("{}", err);
+        exit(1);
     }
 }