make field types a static in Extend

This just adds a new static property called `$field_types` in the Extend model. This let's you do `Extend::$field_types` and print out the available options for the custom fields. Right now it returns text, html, image, file.

It was discussed in #739 on [this comment](https://github.com/anchorcms/anchor-cms/pull/739#issuecomment-58581298) but I think it overlooked in my last PR.