Use blackfriday as markdown parser

examples/markdown.feature

@@ -80,3 +80,16 @@ Feature: Markdown
     """
     When I successfully run `linkcheck foo.md bar.md`
     Then the stdout should contain exactly ""
+
+  Scenario: Check 2 markdown files
+    Given a file named "foo.md" with:
+    """
+    [![Circle CI](https://img.shields.io/circleci/project/github/raviqqe/linkcheck.svg?style=flat-square)](https://circleci.com/gh/raviqqe/linkcheck)
+    [![Go Report Card](https://goreportcard.com/badge/github.com/raviqqe/linkcheck?style=flat-square)](https://goreportcard.com/report/github.com/raviqqe/linkcheck)
+    """
+    And a file named "foo.sh" with:
+    """
+    linkcheck -v foo.md 2>&1 | wc -l
+    """
+    When I successfully run `sh foo.sh`
+    Then the stdout should contain exactly "2"

main.go

@@ -1,15 +1,15 @@
 package main
 
 import (
+	"bytes"
 	"io/ioutil"
 	"net/url"
 	"os"
-	"strings"
 	"time"
 
-	"github.com/a8m/mark"
 	"github.com/docopt/docopt-go"
 	"golang.org/x/net/html"
+	blackfriday "gopkg.in/russross/blackfriday.v2"
 )
 
 func main() {
@@ -50,7 +50,7 @@ func checkFile(c urlChecker, f string) bool {
 		return false
 	}
 
-	n, err := html.Parse(strings.NewReader(mark.Render(string(bs))))
+	n, err := html.Parse(bytes.NewReader(blackfriday.Run(bs)))
 
 	if err != nil {
 		printToStderr(err.Error())