interval type fields not supported

I'm not sure what these should be converted to, but internal isn't a valid type in mysql

```
$ cat test.pgsql
CREATE TABLE public.dcim_device (
id uuid NOT NULL,
created date,
diff_time interval,
);

$ perl pg2mysql.pl < test.pgsql
--
-- Generated by pg2mysql
--
set foreign_key_checks = off;
--
DROP DATABASE IF EXISTS public;
CREATE DATABASE public;
CREATE TABLE public.dcim_device (
`id` varchar(36) NOT NULL,
`created` date,
`diff_time` interval,
);
```

Reproduce the conversion with pg2mysql.pl using the test.pgsql example and inspect how the existing uuid and date fields are handled. Determine a valid MySQL representation for the interval field, then rerun the command and verify that the generated CREATE TABLE no longer contains the unsupported interval type.

Written by the indexing model from the issue text.