Left to add:
All testers are in src/testers.rs
Are there any other ones that seem prudent to add?
divisibleby can probably be added looking at http://jinja.pocoo.org/docs/dev/templates/#builtin-tests.
iterable could be added as well to check if it's an array
It might be better to add a modulo % operator instead of divisibleby.
iterable sounds okay, but then we should add a test for other types, like integer, float, null, etc.
Oh, looks like some of those are in #63 already!