Add relURL template func

Fixes #1126

helpers/url_test.go

@@ -49,6 +49,37 @@ func TestAbsURL(t *testing.T) {
 	}
 }
 
+func TestRelURL(t *testing.T) {
+	defer viper.Set("canonifyURLs", viper.GetBool("canonifyURLs"))
+	tests := []struct {
+		input    string
+		baseURL  string
+		canonify bool
+		expected string
+	}{
+		{"/test/foo", "http://base/", false, "/test/foo"},
+		{"test.css", "http://base/sub", false, "/sub/test.css"},
+		{"test.css", "http://base/sub", true, "/test.css"},
+		{"/test/", "http://base/", false, "/test/"},
+		{"/test/", "http://base/sub/", false, "/sub/test/"},
+		{"/test/", "http://base/sub/", true, "/test/"},
+		{"", "http://base/ace/", false, "/ace/"},
+		{"", "http://base/ace", false, "/ace"},
+		{"http://abs", "http://base/", false, "http://abs"},
+		{"//schemaless", "http://base/", false, "//schemaless"},
+	}
+
+	for i, test := range tests {
+		viper.Set("BaseURL", test.baseURL)
+		viper.Set("canonifyURLs", test.canonify)
+
+		output := RelURL(test.input)
+		if output != test.expected {
+			t.Errorf("[%d][%t] Expected %#v, got %#v\n", i, test.canonify, test.expected, output)
+		}
+	}
+}
+
 func TestSanitizeURL(t *testing.T) {
 	tests := []struct {
 		input    string