Update mm.inc.php

custom_tv type should use textarea, otherwise it is difficult to have " and ' as value in a custom tv. This is important for multiTV and other custom template variables that use json content.

The rules of mm_hidefield use the tv type to hide the tv and multiTV is not hideable. Some other field rules have the issue too.
Merge pull request #4 from Jako/patch-2

custom_tv type should use textarea