Make list option comply with official spec (#112)

Separate page list with newlines, not with commas.

From the official spec:

> Additional decoration MAY be printed if the standard output is a TTY.
> If not, then the output MUST not contain any additional decorations.
> For example a page list MUST be formatted with 1 page name per line
> (to enable easy manipulation using standard CLI tools such as grep etc.).

src/main.rs

@@ -385,7 +385,7 @@ fn main() {
         });
 
         // Print pages
-        println!("{}", pages.join(", "));
+        println!("{}", pages.join("\n"));
         process::exit(0);
     }
 

tests/lib.rs

@@ -6,7 +6,7 @@ extern crate predicates;
 extern crate tempdir;
 extern crate utime;
 
-use std::fs::File;
+use std::fs::{create_dir_all, File};
 use std::io::Write;
 use std::process::Command;
 
@@ -34,6 +34,15 @@ impl TestEnv {
         }
     }
 
+    /// Add entry for that environment.
+    fn add_entry(&self, name: &str, contents: &str) {
+        let dir = self.cache_dir.path().join("tldr-master").join("pages").join("common");
+        create_dir_all(&dir).unwrap();
+
+        let mut file = File::create(&dir.join(format!("{}.md", name))).unwrap();
+        file.write_all(&contents.as_bytes()).unwrap();
+    }
+
     /// Disable default features.
     fn no_default_features(mut self) -> Self {
         self.default_features = false;
@@ -311,3 +320,35 @@ fn test_pager_flag_enable() {
         .assert()
         .success();
 }
+
+#[test]
+fn test_list_flag_rendering() {
+    let testenv = TestEnv::new();
+
+    testenv
+        .command()
+        .args(&["--list"])
+        .assert()
+        .failure()
+        .stderr(contains("Cache not found. Please run `tldr --update`."));
+
+    testenv.add_entry("foo", "");
+
+    testenv
+        .command()
+        .args(&["--list"])
+        .assert()
+        .success()
+        .stdout("foo\n");
+
+    testenv.add_entry("bar", "");
+    testenv.add_entry("baz", "");
+    testenv.add_entry("qux", "");
+
+    testenv
+        .command()
+        .args(&["--list"])
+        .assert()
+        .success()
+        .stdout("bar\nbaz\nfoo\nqux\n");
+}