use a generic Write instance for command output

cli/src/invocation/cmd/version.rs

@@ -1,16 +1,23 @@
 use failure::Fallible;
+use termcolor::{ColorSpec, WriteColor};
 
-pub(crate) fn execute() -> Fallible<()> {
-    println!("TaskChampion {}", env!("CARGO_PKG_VERSION"));
+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"))?;
+    w.reset()?;
     Ok(())
 }
 
 #[cfg(test)]
 mod test {
     use super::*;
+    use crate::invocation::cmd::test::*;
 
     #[test]
     fn test_version() {
-        execute().unwrap();
+        let mut w = test_writer();
+        execute(&mut w).unwrap();
+        assert!(w.into_string().starts_with("TaskChampion "));
     }
 }