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/modify.rs

@@ -0,0 +1,33 @@
+use crate::argparse::{DescriptionMod, Modification};
+use failure::Fallible;
+use taskchampion::TaskMut;
+
+/// Apply the given modification
+pub(super) fn apply_modification(task: &mut TaskMut, modification: &Modification) -> Fallible<()> {
+    match modification.description {
+        DescriptionMod::Set(ref description) => task.set_description(description.clone())?,
+        DescriptionMod::Prepend(ref description) => {
+            task.set_description(format!("{} {}", description, task.get_description()))?
+        }
+        DescriptionMod::Append(ref description) => {
+            task.set_description(format!("{} {}", task.get_description(), description))?
+        }
+        DescriptionMod::None => {}
+    }
+
+    if let Some(ref status) = modification.status {
+        task.set_status(status.clone())?;
+    }
+
+    if let Some(true) = modification.active {
+        task.start()?;
+    }
+
+    if let Some(false) = modification.active {
+        task.stop()?;
+    }
+
+    println!("modified task {}", task.get_uuid());
+
+    Ok(())
+}