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/mod.rs

@@ -0,0 +1,13 @@
+//! Responsible for executing commands as parsed by [`crate::argparse`].
+
+pub(crate) mod add;
+pub(crate) mod gc;
+pub(crate) mod help;
+pub(crate) mod info;
+pub(crate) mod list;
+pub(crate) mod modify;
+pub(crate) mod sync;
+pub(crate) mod version;
+
+#[cfg(test)]
+mod test;