API improvements

Hello,

Great module and you helped me in the past on it, thanks. I can visualize a few small API improvements that would help (beyond the one already requested about per node-type enabling/disabling). After short discussion if these changes are acceptable, I could create a patch for this module.

1. The JS rewrites all links on page load. This is both expensive and very general (as it evaluates all links on the page). The module could provide an API setting and custom module hook to allow a themer the ability to theme only certain page elements or fields (for the use case where only certain links on the page should be targeted). If so desired, the original behaviour is retained (I find for our marketing use-case we need only specific links to be targeted, and site-wide implementation is too much).
2. add a component to the generated internal link: /external?url=http://example.com&referrer=my/referring/page OR /external?url=http://example.com&referrer=node/123
3. add a variable in the template such that a fluid "back link" is generated (checked for url injection)