html/text visibility form

It would be great to have a form similar to the block visibility form that allows you to set which emails should be html and which should remain as plain text. This shouldn't be too difficult as each email has an id that consists of its module name and a key.

There should be two options:
* make all emails html except the following
* only make the following emails html

Also being about to match against patterns like you can with paths:
e.g. modulename_*