Add examples to commands (#1752)

* Pass &dyn WholeStreamCommand to get_help

* Add an optional example to the WholeStreamCommand trait

* Add an example to the alias command

crates/nu-cli/src/commands/command.rs

@@ -350,6 +350,11 @@ impl EvaluatedCommandArgs {
     }
 }
 
+pub struct Example {
+    pub example: &'static str,
+    pub description: &'static str,
+}
+
 pub trait WholeStreamCommand: Send + Sync {
     fn name(&self) -> &str;
 
@@ -368,6 +373,10 @@ pub trait WholeStreamCommand: Send + Sync {
     fn is_binary(&self) -> bool {
         false
     }
+
+    fn examples(&self) -> &[Example] {
+        &[]
+    }
 }
 
 #[derive(Clone)]
@@ -407,7 +416,7 @@ impl Command {
 
     pub fn run(&self, args: CommandArgs, registry: &CommandRegistry) -> OutputStream {
         if args.call_info.switch_present("help") {
-            get_help(self.name(), self.usage(), self.signature(), registry).into()
+            get_help(&*self.0, registry).into()
         } else {
             match self.0.run(args, registry) {
                 Ok(stream) => stream,
@@ -419,6 +428,10 @@ impl Command {
     pub fn is_binary(&self) -> bool {
         self.0.is_binary()
     }
+
+    pub fn stream_command(&self) -> &dyn WholeStreamCommand {
+        &*self.0
+    }
 }
 
 pub struct FnFilterCommand {