Add support for template.URL

Fixes #13

cast_test.go

@@ -37,6 +37,7 @@ func TestToString(t *testing.T) {
 	assert.Equal(t, ToString(8.12), "8.12")
 	assert.Equal(t, ToString([]byte("one time")), "one time")
 	assert.Equal(t, ToString(template.HTML("one time")), "one time")
+	assert.Equal(t, ToString(template.URL("http://somehost.foo")), "http://somehost.foo")
 	assert.Equal(t, ToString(foo), "one more time")
 	assert.Equal(t, ToString(nil), "")
 }

caste.go

@@ -194,6 +194,8 @@ func ToStringE(i interface{}) (string, error) {
 		return string(s), nil
 	case template.HTML:
 		return string(s), nil
+	case template.URL:
+		return string(s), nil
 	case nil:
 		return "", nil
 	case fmt.Stringer: