Allow functions in addition to maps for partials

[jcromartie]

I would like to be able to pass a function that takes a partial name and returns the template, instead of a map of names to partial templates. Using a memoized function would enable lazy loading of partial resources rather than having to load every partial that a template may use ahead of time.

[benfleis]

+1 as I just implemented the annoying (load-partials) to preload everything :/