Fix clippy warnings and make them all errors

cli/src/invocation/cmd/add.rs

@@ -13,7 +13,7 @@ pub(crate) fn execute<W: WriteColor>(
         _ => "(no description)".to_owned(),
     };
     let t = replica.new_task(Status::Pending, description).unwrap();
-    write!(w, "added task {}\n", t.get_uuid())?;
+    writeln!(w, "added task {}", t.get_uuid())?;
     Ok(())
 }
 

cli/src/invocation/cmd/gc.rs

@@ -4,7 +4,7 @@ use termcolor::WriteColor;
 
 pub(crate) fn execute<W: WriteColor>(w: &mut W, replica: &mut Replica) -> Fallible<()> {
     replica.gc()?;
-    write!(w, "garbage collected.\n")?;
+    writeln!(w, "garbage collected.")?;
     Ok(())
 }
 

cli/src/invocation/cmd/sync.rs

@@ -8,7 +8,7 @@ pub(crate) fn execute<W: WriteColor>(
     server: &mut Box<dyn Server>,
 ) -> Fallible<()> {
     replica.sync(server)?;
-    write!(w, "sync complete.\n")?;
+    writeln!(w, "sync complete.")?;
     Ok(())
 }
 

cli/src/invocation/cmd/version.rs

@@ -4,7 +4,7 @@ use termcolor::{ColorSpec, WriteColor};
 pub(crate) fn execute<W: WriteColor>(w: &mut W) -> Fallible<()> {
     write!(w, "TaskChampion ")?;
     w.set_color(ColorSpec::new().set_bold(true))?;
-    write!(w, "{}\n", env!("CARGO_PKG_VERSION"))?;
+    writeln!(w, "{}", env!("CARGO_PKG_VERSION"))?;
     w.reset()?;
     Ok(())
 }