Save generated changelog to file

[DariuszDepta]

Add option -o --output to changelog subcommand that takes a file name as an argument and saves the generated changelog to the output file.

[DariuszDepta]

Here is the place where the changelog is printed to stdout:

magg/src/cli.rs

Lines 168 to 172 in 7e13fe7

	Ok(changelog) => {
	println!("\nCHANGELOG");
	println!("{SEPARATOR_LINE}");
	println!("{}", changelog)
	}

After printing to stdout, the same content should be written to output file (just the changelog variable in line 171 without the header like in stdout).