Make tagKey an exported var to be able to use another struct tag

[abemedia]

Checklist

• I have searched the issue list
• I have tested my example against Shopify Liquid. (This isn't necessary if the actual behavior is a panic, or an error for which IsTemplateError returns false.)

Detailed Description

I want to be able to use liquid on structs from other packages, where I have no control over the struct tags. Specifically I'd like to use the json tag.

Possible Solution

Amend the tagKey constant to be an exported var:

liquid/values/structvalue.go

Line 66 in d27c839

const tagKey = "liquid"