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/add.rs

@@ -0,0 +1,34 @@
+use crate::argparse::{DescriptionMod, Modification};
+use failure::Fallible;
+use taskchampion::{Replica, Status};
+
+pub(crate) fn execute(replica: &mut Replica, modification: Modification) -> Fallible<()> {
+    let description = match modification.description {
+        DescriptionMod::Set(ref s) => s.clone(),
+        _ => "(no description)".to_owned(),
+    };
+    let t = replica.new_task(Status::Pending, description).unwrap();
+    println!("added task {}", t.get_uuid());
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::cmd::test::test_replica;
+
+    #[test]
+    fn test_add() {
+        let mut replica = test_replica();
+        let modification = Modification {
+            description: DescriptionMod::Set("my description".to_owned()),
+            ..Default::default()
+        };
+        execute(&mut replica, modification).unwrap();
+
+        // check that the task appeared..
+        let task = replica.get_working_set_task(1).unwrap().unwrap();
+        assert_eq!(task.get_description(), "my description");
+        assert_eq!(task.get_status(), Status::Pending);
+    }
+}