feat: Refactor hint system

Hints are now accessible using the CLI subcommand `rustlings hint
<exercise name`.

BREAKING CHANGE: This fundamentally changes the way people interact with exercises.

src/exercise.rs

@@ -27,6 +27,7 @@ pub struct Exercise {
     pub name: String,
     pub path: PathBuf,
     pub mode: Mode,
+    pub hint: String,
 }
 
 impl Exercise {
@@ -74,6 +75,7 @@ mod test {
             name: String::from("example"),
             path: PathBuf::from("example.rs"),
             mode: Mode::Test,
+            hint: String::from(""),
         };
         exercise.clean();
         assert!(!Path::new(&temp_file()).exists());

src/main.rs

@@ -27,6 +27,12 @@ fn main() {
                 .about("Runs/Tests a single exercise")
                 .arg(Arg::with_name("name").required(true).index(1)),
         )
+        .subcommand(
+            SubCommand::with_name("hint")
+                .alias("h")
+                .about("Returns a hint for the current exercise")
+                .arg(Arg::with_name("name").required(true).index(1)),
+        )
         .get_matches();
 
     if None == matches.subcommand_name() {
@@ -71,6 +77,20 @@ fn main() {
         run(&exercise).unwrap_or_else(|_| std::process::exit(1));
     }
 
+    if let Some(ref matches) = matches.subcommand_matches("hint") {
+        let name = matches.value_of("name").unwrap_or_else(|| {
+            println!("Please supply an exercise name!");
+            std::process::exit(1);
+        });
+
+        let exercise = exercises.iter().find(|e| name == e.name).unwrap_or_else(|| {
+            println!("No exercise found for your given name!");
+            std::process::exit(1)
+        });
+
+        println!("{}", exercise.hint);
+    }
+
     if matches.subcommand_matches("verify").is_some() {
         verify(&exercises).unwrap_or_else(|_| std::process::exit(1));
     }