Typekit on non-cam domains

[alt36]

(I'm unsure if this is best dealt with here or the parent Project Light repo; because the change I'm proposing would be implemented well inside Drupal's theming layers this feels like the right place though)

I've a website running PL but on a non-cam domain, which means that by default I don't get the typekit fonts loading. The UIS have helpfully created a new webkit for me to facilitate this, which means I need to modify the src for the typekit-hosted js in html.tpl.php . I could do this in my subtheme of PL, but that seems distinctly sub-optimal (not least of all because I'd have to create a standalone theme for the sake of one site)

A better solution would seem to be to add the typekit ID as a theme setting: I'm happy to prepare a small PR if it's agreed that that's the best approach.