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/version.rs

@@ -0,0 +1,16 @@
+use failure::Fallible;
+
+pub(crate) fn execute() -> Fallible<()> {
+    println!("TaskChampion {}", env!("CARGO_PKG_VERSION"));
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_version() {
+        execute().unwrap();
+    }
+}