Allow loop modifiers to be more than just literals

[aisbergg]

This PR adds the ability to specify variables and expressions in loop modifiers. Example:

{% assign limit = 3 %}
{% assign offset = 1 %}
{% for i in (1..5) limit:limit offset:offset %}
{{ i -}}
{%- endfor %}

results in:

2
3
4

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes.
• [x] New and changed code is covered by tests.
• [ ] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.