Add variable support (`{% include {{ page.my_variable }} %}`) to our Jekyll-style `include`

[epage]

Depends on #309

In looking at the Jekyll docs, it appears shopify allows expressions for include.

https://jekyllrb.com/docs/includes/

Besides the parser (see #138), we'd have a problem with this because we inline includes when parsing the file.

#95 steps through some of the design trade offs we might have to go through to fix this.

[epage]

How needed is this?

I don't see it in shopify's proprietary documentation https://help.shopify.com/themes/liquid/tags/theme-tags#include