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/help.rs

@@ -0,0 +1,28 @@
+use failure::Fallible;
+
+pub(crate) fn execute(command_name: String, summary: bool) -> Fallible<()> {
+    println!(
+        "TaskChampion {}: Personal task-tracking",
+        env!("CARGO_PKG_VERSION")
+    );
+    if !summary {
+        println!();
+        println!("USAGE: {} [args]\n(help output TODO)", command_name); // TODO
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_summary() {
+        execute("task".to_owned(), true).unwrap();
+    }
+
+    #[test]
+    fn test_long() {
+        execute("task".to_owned(), false).unwrap();
+    }
+}