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/invocation/cmd/test.rs

@@ -0,0 +1,14 @@
+use taskchampion::{server, taskstorage, Replica, ServerConfig};
+use tempdir::TempDir;
+
+pub(super) fn test_replica() -> Replica {
+    let storage = taskstorage::InMemoryStorage::new();
+    Replica::new(Box::new(storage))
+}
+
+pub(super) fn test_server(dir: &TempDir) -> Box<dyn server::Server> {
+    server::from_config(ServerConfig::Local {
+        server_dir: dir.path().to_path_buf(),
+    })
+    .unwrap()
+}