Allow multiple external links and also support internal urls

This patch does 2 things:
- Allows urls from the internal site to be put also in iframes. This is useful if you have a pdf or other document you'd like to be put within an iframe. This can be turned on in the settings. One thing to note, it's probably not a good idea to rewrite all urls AND turn this on as it will make all of your links open in iframes.

- You can now have different templates for each url. There are 2 ways to accomplish this:
-- Add the data-externaliframe attribute to your links: <a href="http://google.com" data-externaliframe="test">Google</a>. The url from this will be external/test?url=http://google.com. A template with the name of page--external--test.tpl.php will be used if present, otherwise, it will fall back to the default template.
-- Hard code the link you'd like. Put whatever values after /external that you'd like and the corresponding template will be used.

Hope this is helpful to someone else.