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/info.rs

@@ -0,0 +1,49 @@
+use crate::argparse::Filter;
+use crate::invocation::filtered_tasks;
+use crate::table;
+use failure::Fallible;
+use prettytable::{cell, row, Table};
+use taskchampion::Replica;
+
+pub(crate) fn execute(replica: &mut Replica, filter: Filter, debug: bool) -> Fallible<()> {
+    for task in filtered_tasks(replica, &filter)? {
+        let uuid = task.get_uuid();
+
+        let mut t = Table::new();
+        t.set_format(table::format());
+        if debug {
+            t.set_titles(row![b->"key", b->"value"]);
+            for (k, v) in task.get_taskmap().iter() {
+                t.add_row(row![k, v]);
+            }
+        } else {
+            t.add_row(row![b->"Uuid", uuid]);
+            if let Some(i) = replica.get_working_set_index(uuid)? {
+                t.add_row(row![b->"Id", i]);
+            }
+            t.add_row(row![b->"Description", task.get_description()]);
+            t.add_row(row![b->"Status", task.get_status()]);
+            t.add_row(row![b->"Active", task.is_active()]);
+        }
+        t.printstd();
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::invocation::cmd::test::test_replica;
+
+    #[test]
+    fn test_info() {
+        let mut replica = test_replica();
+        let filter = Filter {
+            ..Default::default()
+        };
+        let debug = false;
+        execute(&mut replica, filter, debug).unwrap();
+        // output is to stdout, so this is as much as we can check
+    }
+}