Add command to show recommendations. (#593)

* Add command to show recommendations.

This adds a command `similar selected|current` which enables searching for
track recommendations for playlists, albums as well as single tracks.

* Make sure to only send 5 seed items in total.

* Add docs for recommendation bindings to the README

src/commands.rs

@@ -258,6 +258,7 @@ impl CommandManager {
             | Command::Delete
             | Command::Back
             | Command::Open(_)
+            | Command::ShowRecommendations(_)
             | Command::Insert(_)
             | Command::Goto(_) => Ok(None),
             _ => Err("Unknown Command".into()),
@@ -390,6 +391,15 @@ impl CommandManager {
         kb.insert("a".into(), Command::Goto(GotoMode::Album));
         kb.insert("A".into(), Command::Goto(GotoMode::Artist));
 
+        kb.insert(
+            "m".into(),
+            Command::ShowRecommendations(TargetMode::Selected),
+        );
+        kb.insert(
+            "M".into(),
+            Command::ShowRecommendations(TargetMode::Current),
+        );
+
         kb.insert("Up".into(), Command::Move(MoveMode::Up, Default::default()));
         kb.insert(
             "p".into(),