Allow Customizable URL Trim Length

The module defaults to 72 for URL lengths - any longer and it trims it and adds a "...". This breaks for a couple of mail clients (my school uses SquirrelMail a lot, and it doesn't read HTML mail properly so copy-pasting a link longer than 72 characters doesn't work). I suggest that the length be customizable through the admin/settings/htmlmail page as follows (using "0" to disable trimming):

• Add the field to the settings page
• In the _htmlmail_url_trim function, replace "72" with the appropriate variable_get
• In _htmlmail_url_trim, add $length != 0 && to the second if statement.

I've included a patch to the latest CVS snapshot for review; I'm not entirely sure about my wording for the URL Length field description.