use a generic Write instance for command output

cli/src/invocation/cmd/help.rs

@@ -1,24 +1,31 @@
 use crate::usage::Usage;
 use failure::Fallible;
-use std::io;
+use termcolor::WriteColor;
 
-pub(crate) fn execute(command_name: String, summary: bool) -> Fallible<()> {
+pub(crate) fn execute<W: WriteColor>(
+    w: &mut W,
+    command_name: String,
+    summary: bool,
+) -> Fallible<()> {
     let usage = Usage::new();
-    usage.write_help(io::stdout(), command_name, summary)?;
+    usage.write_help(w, command_name, summary)?;
     Ok(())
 }
 
 #[cfg(test)]
 mod test {
     use super::*;
+    use crate::invocation::cmd::test::*;
 
     #[test]
     fn test_summary() {
-        execute("task".to_owned(), true).unwrap();
+        let mut w = test_writer();
+        execute(&mut w, "task".to_owned(), true).unwrap();
     }
 
     #[test]
     fn test_long() {
-        execute("task".to_owned(), false).unwrap();
+        let mut w = test_writer();
+        execute(&mut w, "task".to_owned(), false).unwrap();
     }
 }