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/gc.rs

@@ -0,0 +1,21 @@
+use failure::Fallible;
+use taskchampion::Replica;
+
+pub(crate) fn execute(replica: &mut Replica) -> Fallible<()> {
+    replica.gc()?;
+    println!("garbage collected.");
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::cmd::test::test_replica;
+
+    #[test]
+    fn test_gc() {
+        let mut replica = test_replica();
+        execute(&mut replica).unwrap();
+        // this mostly just needs to not fail!
+    }
+}