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")
+}