feat: add unit tests

t/08-horizontal-rules.t

@@ -0,0 +1,13 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use Test::More tests => 3;
+use MarkdownParser;
+
+my $parser = MarkdownParser->new();
+
+is( $parser->parse("---"), "<hr>\n", "Horizontal rule with ---" );
+is( $parser->parse("***"), "<hr>\n", "Horizontal rule with ***" );
+is( $parser->parse("___"), "<hr>\n", "Horizontal rule with ___" );
+