Adding textarea option for theme settings.

Signed-off-by: ichbin <ichbin@ichbin.us>

In light of this request - http://www.elkarte.net/index.php?topic=863 it got me thinking.

I was looking at the theme options, and noticed we can only have a textbox to insert custom code into a theme. Any reason we couldn't allow a textarea so that someone could use the box to insert some custom code into a theme, without them having to edit the theme? I realize that a textbox can do the same thing, but you can't see all the code and you can't resize the textbox to view it like a textarea. :)