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/sync.rs

@@ -0,0 +1,26 @@
+use failure::Fallible;
+use taskchampion::{server::Server, Replica};
+
+pub(crate) fn execute(replica: &mut Replica, server: &mut Box<dyn Server>) -> Fallible<()> {
+    replica.sync(server)?;
+    println!("sync complete.");
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::cmd::test::{test_replica, test_server};
+    use tempdir::TempDir;
+
+    #[test]
+    fn test_add() {
+        let mut replica = test_replica();
+        let server_dir = TempDir::new("test").unwrap();
+        let mut server = test_server(&server_dir);
+
+        // this just has to not fail -- the details of the actual sync are
+        // tested thoroughly in the taskchampion crate
+        execute(&mut replica, &mut server).unwrap();
+    }
+}