Support CLI syntax for changing tags

cli/src/invocation/modify.rs

@@ -1,5 +1,6 @@
 use crate::argparse::{DescriptionMod, Modification};
 use failure::Fallible;
+use std::convert::TryInto;
 use taskchampion::TaskMut;
 use termcolor::WriteColor;
 
@@ -32,6 +33,18 @@ pub(super) fn apply_modification<W: WriteColor>(
         task.stop()?;
     }
 
+    for tag in modification.add_tags.iter() {
+        // note that the parser should have already ensured that this tag was valid
+        let tag = tag.try_into()?;
+        task.add_tag(&tag)?;
+    }
+
+    for tag in modification.remove_tags.iter() {
+        // note that the parser should have already ensured that this tag was valid
+        let tag = tag.try_into()?;
+        task.remove_tag(&tag)?;
+    }
+
     write!(w, "modified task {}\n", task.get_uuid())?;
 
     Ok(())