Support fetching READMEs from GitHub

When passing a valid GitHub URL instead of a filename to gold,
it will fetch either README.md or README (in that order) from
GitHub and displays the markdown content.

Example:

./gold -s dark.json https://github.com/muesli/beehive

github.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"errors"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+// isGitHubURL tests a string to determine if it is a well-structured GitHub URL
+func isGitHubURL(s string) bool {
+	u, err := url.ParseRequestURI(s)
+	if err != nil {
+		return false
+	}
+
+	return strings.ToLower(u.Host) == "github.com"
+}
+
+// findGitHubREADME tries to find the correct README filename in a repository
+func findGitHubREADME(s string) (*http.Response, error) {
+	u, err := url.ParseRequestURI(s)
+	if err != nil {
+		return nil, err
+	}
+
+	u.Host = "raw.githubusercontent.com"
+	readmeNames := []string{"README.md", "README"}
+
+	for _, r := range readmeNames {
+		v := u
+		v.Path += "/master/" + r
+
+		resp, err := http.Get(v.String())
+		if err != nil {
+			return nil, err
+		}
+
+		if resp.StatusCode == http.StatusOK {
+			return resp, nil
+		}
+	}
+
+	return nil, errors.New("can't find README in GitHub repository")
+}

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
 
@@ -17,13 +18,29 @@ func main() {
 		fmt.Println("Missing Markdown file. Usage: ./gold -s STYLE.json FILE.md")
 		os.Exit(1)
 	}
-	f, err := os.Open(args[0])
-	if err != nil {
-		fmt.Println(err)
-		os.Exit(1)
+
+	var in io.Reader
+	if isGitHubURL(args[0]) {
+		resp, err := findGitHubREADME(args[0])
+		if err != nil {
+			fmt.Println(err)
+			os.Exit(1)
+		}
+
+		defer resp.Body.Close()
+		in = resp.Body
+	} else {
+		f, err := os.Open(args[0])
+		if err != nil {
+			fmt.Println(err)
+			os.Exit(1)
+		}
+
+		defer f.Close()
+		in = f
 	}
-	defer f.Close()
-	b, _ := ioutil.ReadAll(f)
+
+	b, _ := ioutil.ReadAll(in)
 	out, err := gold.RenderBytes(b, *s)
 	if err != nil {
 		fmt.Println(err)