Limit `for` iterations: request for comments

[vgrigoriu]

We're going to use Pebble with untrusted templates. Among other measures we'd like to add a limit to the number of iterations that happen in a {% for %}, to guard against infinite / very long loops.

@ebussieres, do you think this is something that might be useful for other Pebble users? Would it make sense to open a PR here or should we keep it to ourselves?