firstof filter (return the first true value of a list)

The firstof filter return the first element that is evaluated to True in
a sequence, it return '' if all elements are evaluated to False.

How to use ?
============

{{ ['hello', baz]|firstof }} -> 'hello'
{{ [0, False]|firstof }} -> ''
{{ [0, False, foo]|firstof }} with foo='bar' -> 'bar'

It's inspired by django firstof:
https://docs.djangoproject.com/en/dev/ref/templates/builtins/?from=olddocs#firstof