Please add support for enums
I have no plans at present to support this. It should be easy, if you so desire, to implement an `EnumField` as a separate class.
Postgresql allows to create enum-types like

```
CREATE TYPE colors AS ENUM ('red', 'green', 'blue');
```

which is quite useful. Its a feature i really would like to see supported in peewee.

Thank you! :) 