Add support for template.URL

Fixes #13

cast_test.go

diff --git a/cast_test.go b/cast_test.go
index 3fa717f..a202b7b 100644
--- a/cast_test.go
+++ b/cast_test.go
@@ -37,6 +37,7 @@
 	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

diff --git a/caste.go b/caste.go
index 58d72df..9ed19ad 100644
--- a/caste.go
+++ b/caste.go
@@ -194,6 +194,8 @@
 		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: