Add where/reject option to for

[bakura10]

Hi,

In Shopify, we often need to loop through blocks and ignore some depending on conditions. It would be very handy if for loops would support an extra "where" and "reject" filters:

{%- for block in section.blocks where: 'settings.image' -%}
  Only loop through blocks whose image is defined 
{%- else -%}
  Show placeholder
{%- endfor -%}

This would make the syntax much more expressive and would remove a lot of useless creation of temporary variables (which are often source of hard to track bug when they override already existing variables).