Allow any tag/block to store user-readable state

[epage]

Currently, increment / decrement have special support in Context. We should generalize this to be like the ruby version so any plugin can do this.

[epage]

They use environments

https://github.com/Shopify/liquid/blob/master/lib/liquid/tags/increment.rb#L24 https://github.com/Shopify/liquid/blob/master/lib/liquid/context.rb#L163

Not sure when the squash is used https://github.com/Shopify/liquid/blob/master/lib/liquid/context.rb#L215