feat(changelog): add output flag

Use `convco changelog -o CHANGELOG.md` to write the output to the file instead of stdout.

src/cli.rs

@@ -130,6 +130,9 @@ pub struct ChangelogCommand {
     /// If a semantic version is given, the title will be prefixed.
     #[clap(short, long, default_value = "Unreleased")]
     pub unreleased: String,
+    /// Path to write the changelog to.
+    #[clap(short, long, default_value = "-")]
+    pub output: PathBuf,
 }
 
 #[derive(Debug, Parser)]

src/cmd/changelog.rs

@@ -391,8 +391,11 @@ impl ChangelogCommand {
 
 impl Command for ChangelogCommand {
     fn exec(&self, config: Config) -> anyhow::Result<()> {
-        let stdout = std::io::stdout().lock();
-        self.write(config, stdout)?;
+        let out: Box<dyn Write> = match self.output.as_path() {
+            p if p.to_string_lossy() == "-" => Box::new(std::io::stdout().lock()),
+            p => Box::new(std::fs::File::create(p)?),
+        };
+        self.write(config, out)?;
         Ok(())
     }
 }