feat: Index exercises by name

BREAKING CHANGE: This changes the way you use `rustlings run` by now
requiring an abridged form of the previous filename, e.g:

`rustlings run exercises/if/if1.rs` becomes
`rustlings run if1`

src/main.rs

@@ -25,8 +25,7 @@ fn main() {
             SubCommand::with_name("run")
                 .alias("r")
                 .about("Runs/Tests a single exercise")
-                .arg(Arg::with_name("file").required(true).index(1))
-                .arg(Arg::with_name("test").short("t").long("test").help("Run the file as a test")),
+                .arg(Arg::with_name("name").required(true).index(1)),
         )
         .get_matches();
 
@@ -55,20 +54,17 @@ fn main() {
     let exercises = toml::from_str::<ExerciseList>(toml_str).unwrap().exercises;
 
     if let Some(ref matches) = matches.subcommand_matches("run") {
-        let filename = matches.value_of("file").unwrap_or_else(|| {
-            println!("Please supply a file name!");
+        let name = matches.value_of("name").unwrap_or_else(|| {
+            println!("Please supply an exercise name!");
             std::process::exit(1);
         });
 
         let matching_exercise = |e: &&Exercise| {
-            Path::new(filename)
-                .canonicalize()
-                .map(|p| p.ends_with(&e.path))
-                .unwrap_or(false)
+            name == e.name
         };
 
         let exercise = exercises.iter().find(matching_exercise).unwrap_or_else(|| {
-            println!("No exercise found for your file name!");
+            println!("No exercise found for your given name!");
             std::process::exit(1)
         });