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/macros.rs

@@ -0,0 +1,12 @@
+#![macro_use]
+
+/// create a &[&str] from vec notation
+#[cfg(test)]
+macro_rules! argv {
+    () => (
+        &[][..]
+    );
+    ($($x:expr),* $(,)?) => (
+        &[$($x),*][..]
+    );
+}