Support slice shorthands

[olets]

Twig has the shorthand ...[start:length] for |slice(start, length). In the shorthand form, either value can be omitted (...[1:2], ...[:2], and ...[2:] are supported; start defaults to 0, length defaults to until-the-end).

It'd be great to have support for these!

See slice: http://twig.sensiolabs.org/doc/filters/slice.html